Why was Mariam shocked to learn she would bear Jesus?

In the Gospel of Luke a virgin betrothed to Joseph was visited by the angel or messenger Gabriel. He announced that she would bear a son to be named Jesus. 

She was shocked. She said in Luke 1:35 'How shall this be, seeing I know not a man?' 

But she was 'betrothed' to Joseph! The verb 'know' must mean have sexual relations with him. 

Isn't that peculiar? Did it need an angel to tell her that marriage leads to children?

The reply of Gabriel is also unusual. It is not what we would expect.

'The holy spirit will come upon you and the power of the Highest shall overshadow you. Therefore the holy born one shall be called the son of God. And behold, your cousin Elizabeth. She also conceived a son in her old age. And this is the sixth month with her that was called barren. For with God, nothing is impossible.' 
Does this expression 'the holy spirit will come upon you' have anything to do with insemination of seed so Joseph was sidelined and rendered redundant? Hardly. A few verses later we learn in 2:25 that the holy spirit 'was upon' someone else. Who? Simeon the retired high priest! He was old and hardly pregnant! 

Was Mary 14 years old?
If as many churches say, Mary -- or Mariam as it is in the Greek NT -- was 14 years of age, why is she so surprised that one day in her marriage she will become pregnant. And what was Joseph doing marrying a 14 year old? Isn't that child abuse? Nothing in scripture supports a teenage girl. The only source of this is pagan mythologies where unmarried, unbetrothed virgins are inseminated by Jupiter or some other god. 
Why does the angel mention an old, grey or white haired cousin called Elizabeth? The word for old age here is the Greek, gera, from which we derive our term geriatrics. Cousins are usually in the same age bracket. This is confirmed in the Greek text. Note the word 'also'.

The Greek says:
And behold, (idou -- Just take a look at that!) your cousin Elizabeth! She also conceived a son in her old age. And this is the sixth month with her that was called barren. For with God, nothing is impossible.' (Or more correctly 'Also she conceived in her old age.' 

Luke also uses the word idou in chapter 2: 25 meaning Surprise, surprise! when Joseph and Mariam meet Simeon the former high priest in the Temple. He was forcibly retired by the paranoid and murderous Herod the Great. He is Mariam's relative. 
For reasons of self-preservation in those dangerous Herodian times they lived quite separately without contact.

'Behold Elizabeth! Also she conceived in her old age.' The 'also' of Luke 1 verse 35 must refer to a miracle relating to either pregnancy or old age ... or both. If Mariam was a young girl or even in her twenties it would not be a miracle that she would become pregnant as announced. A married Jewish woman would expect to have children as part of the reasons for marriage. In her case much more so as Joseph was inheritor of the house of David and her son would be a royal born at a prophetic time according to Daniel when many expected the Messiah. It had obviously not happened, much to her disappointment. Mariam was not some unknown woman but esteemed in her own right as a direct descendant of Aaron, with an authentic genealogy.

If she were in her 20s she should still expect to have children.

Let's make the first correction. Instead 'man' she is asking: 

'How can this be since I have not known (had relations) with my husband?' 

The Greek word here 'andra' does not mean 'man' in the sense of 'any man'. 'I have not been with a man.' That's what the RCC-Protestants say. They say she was a young girl engaged to Joseph but didn't know a man. Was she looking for other men? The text says in KJV she was 'betrothed'! Would she say to the angel that she was looking for someone else to have sex with? That's a crazy and blasphemous idea. 
Engagement

Non-Jewish sectarians are patently wrong because they fail to understand -- or do not want to understand -- what were Jewish marriages/ weddings customs. The answer is simple. They could crack open a Jewish encyclopedia or ask a local Jew at the synagogue. That is not too difficult. Instead they transpose Roman or even loose, modern ideas about 'engagement'. Not so.

Mariam was a daughter of Aaron -- a highly esteemed priestly family with only a few survivors. She was married to a royal son of the House of David. This took place according to ancient custom. Even common people of Israel in first century Judea and elsewhere around the world, followed these customs, according to Josephus and Philo. They kept registers in all the global depositories showing these customs of betrothal, wedding, birth were respected, with names of witnesses. Jews do this today. If a child of a man named Cohen = priest in Hebrew) does not conform to Hebrew rules of marriage, then all his offspring for ever cannot be considered viable as priests.

This regulation was essential if children were to be considered fit for marriage as priests and the same goes for kings and tribal leaders. Ten generations had to be verified by these interlocking methods where many independent registries confirm ancient families.

Betrothal
The first question to ask is: what did the Hebrew or first century Jews mean by 'betrothal'? Qiddushin or Betrothal in the Hebrew or Jewish sense gave the couple the right to live together with all rights of sexual relationship. If either man or wife wanted to separate, a divorce called a 'get' was needed before a judge.

So the shock reply of Mariam: 'How can this be?!' to a pregnancy has to be explained.

We further learn that she was still a virgin! In Jewish custom the term primarily has to do with menstruation. So when a woman says she is still a virgin it means that she was not menstruating or had not menstruated. Amongst the priestly class, no intercourse could take place until a wife had had three successive, regular monthly periods to show she was in good health. That's in Josephus and other sources of the time. The Talmud tractate Niddah written a few centuries later also says that a woman who was old and undergone the change and ceased to menstruate could also be considered a virgin.

Virgin
So the reason for her shock must be (a) she was 11 or 12 and had not menstruated and was naive and thought she never would, (b) she was married a long time she had not had regular periods, and Joseph had never 'known' her, or (c) she was old and had had her menopause some long while ago. Her infertility was certain to her.

Two other points. Firstly, context. The whole of Luke's first two long chapters is about women in their old age. In chapter one we have her cousin, Elizabeth/ Elisheva who was 'stricken in years'. Then we have Mariam / Miriam. The sister of Moses, Miriam the prophetess, married in her old age, Caleb, according to Jewish tradition. She had a flourishing and important family. Then in chapter two we have Anna or Hanna in Hebrew the prophetess of the tribe of Asher. Hanna as a name recalls that of the mother of Samuel the prophet who gave birth in old age, miraculously after much prayer. The NT Anna or Hanna was either 84 years old or much older depending how you read the text. She was 'advanced in years' v 36 having lived with her husband from her virginity seven years. That seems to confirm the Niddah laws of the Hebrew scriptures (and Talmud) are in force.

Secondly we can make a mathematical calculation of Mariam's age. I did this in section 15.17 of the book, Jesus, James, Joseph. It is based on Eusebius. After James the brother of Jesus was killed, Symeon the cousin of Jesus was made the Bishop / Supervisor or Governor/ Sagan of the ekklesia at Jerusalem. He was a younger son of of the younger brother of Joseph called Cleopas or Alpheus. Eusebius gives us his age at his death. We do not know the age of Maria, the wife of Cleopas when she started having children. But the calculation shows that Mariam was old when she had Jesus and later James, Jude, Simeon, Joses and probably three or so daughters.

Mariam was obviously old and past her menopause. That is why she was surprised at the announcement of a birth of a son to come.

Tom Paine debunks the Virgin Birth. What else did he miss?

Is the belief in Mary's Virgin birth is necessary for salvation? The answer to the question is "It depends on what you mean by Virgin Birth".  The conundrum arises because people today seldom read the Bible logically. They confuse what the NT actually says with common custom of "Christmas", nativity and Virgin Birth (that is virginal conception). There are animals and stables, mangers and the story of 'no room for them in the Inn'.
This mythology derives from the non-Jewish and often paganized church of the third and fourth centuries. The dogma of the BVM (Blessed Virgin Mary) is maintained up to the present by Roman Catholics, Protestants and others.
The 18th century best-selling writer and revolutionary,Thomas Paine, opposed this false interpretation. Fine! but he was fighting many shadows and falsehoods. The paganized accretions of the church also added quite a number of falsities that are still present today.
Mary's name was not Mary -- it is Mariam in the Greek NT. This is equivalent to the name of the sister of Moses, a Levite. She was not a young girl but was an old woman like her cousin, Elizabeth. And Elizabeth was equivalent to the Jewish name, Elisheva, Aaron's wife.
Mariam was married and still a virgin according to Hebrew definitions. There is no stable in the NT. There is no inn. There is no cave. The city of David, Bethlehem, was not full of people. It was probably empty except for a few shepherds. (Claiming descent from David was a death sentence in usurper 'King' Herod's regime. No one was even called David in NT times!) No animals, and no stable. But Mariam did not give birth alone. That would have invalidated the genealogy. She needed at least two or three proven honest witnesses.
These false accretions came from Mithraism that was absorbed into Roman religion around the first century. The RC Virgin Birth came from classical pantheism were the gods descended and took human virgins to procreate the demigods.
The Magi did not come at the birth but a year or two later. And there were not three, there were probably hundreds or thousands of Parthian Magi in a caravan that caused the whole of Jerusalem to tremble with fear of another Roman-Parthian war. Parthia had defeated Rome in numerous times in bloody wars and a peace treaty had been made less than 20 years earlier.
And as Paine rightly points out this BVM business adds impossibility to improbability and the illegitimacy of the RC story. In fact it demolishes it. It requires patience and logic to discover and expose lies and falsehoods. A contemporary of Paine was the famous scientist Joseph Priestley. Like many of the scientists of his day, he wrote a huge amount of theology, as in the age of Reason, the logical methodology required was the same. Paine was not up to par on reason. I recommend Priestley's books such as "History of the Corruptions of Christianity" 1782, 'History of early Opinions concerning Jesus Christ'', 'Discourses on Evidences of Revealed Religion' (delivered at the church of the Universalists, Philadelphia 1796 and dedicated to John Adams).
What is important is less the details of history but a willingness to find the truth, for example about the Resurrection and the NT as a whole and act on it.

Tom Paine is considered a hero of American Independence. Born in England, he became an American citizen and then in France during the Revolution was elected to the French Assembly! He wrote a number of books like Common Sense, The Rights of Man, and the Age of Reason, that were best sellers of his time. They still sell well today.
He was raised on the Bible and was able to expose quite a bit of the hypocrisy of the religion of his day. He described himself as a Deist and his rejection of traditional beliefs alienated him from many of his contemporaries.
He was also in conversation with scholars after he wrote this book, so we don't know his final thoughts. He was right to ask questions. But not all of his conclusions are correct. What is useful is the effort to separate the original truth about Christ and the later traditions of men and politicians.
If you consider the so-called Christian traditions of the RC-Protestants as a series of falsehoods overlaid on the biblical account, Paine exposed and removed some of the most pertinent. He said about the Virgin Birth in the Age of Reason that:

Were any girl that is now with child to say, and even if to swear it, that she was gotten with child by a ghost, and an angel told her so, would she be believed?  Certainly she would not.  Why, then, are we to believe the same thing of another girl, whom we never saw, told by nobody knows who, nor when, nor where?  How strange and inconsistent it is, that the same circumstance that would weaken the belief even of a probable story, should be given as a motive for believing this one, that has upon the face of it every token of absolute impossibility and imposture!"

No one would believe her then. No one should believe her today. Yet millions are taught this unbelievable and unprovable claim by a teenager as the foundation for religious belief and obedience to a denomination. Some blasphemously call this girl 'the mother of god.' That is a pagan title of the pagan Queen of Heaven.
Where did the Virgin Birth come from? Why is it held so firmly? The overlay here is ancient and pagan. It is that of the long-held, Greco-Roman gods impregnating human virgins. 
How did paganism get mixed up with Christianity? Thank the fourth century imperial church of Emperor Constantine for this mish-mash! Paine is right to show that such assertions don't make sense. They render the improbable impossible. He exposes it as an imposture. He was a free man and spoke his mind. In the Middle Ages the imperial church would have required his life. In fact Paine almost lost his life for his frankness.
The fraudulent doctrine comes by combining (syncretizing) current beliefs to make a new imperial religion acceptable to all. The imperial church and the popes gave 'Christian' names to the pagan gods. But this allegation of an unmarried teenager claiming a Virgin conception by a pagan-style god is at odds with the Bible.
The NT says the opposite. Jesus had a human father. Mariam says to Jesus about Joseph: 'Your father and I have been searching for you.' The disciples say that Jesus is the son of Joseph. Did the popes know better than the real mother and father?
But wasn't Jesus also divine? Where Paine does not analyze it is in the matter of a second birth mentioned to Nikodemus in John 3. This begettal occurred at the baptism of Jesus as an adult human. It was physically seen as 'the form of a dove'.
A further overlay of lies or ignorance that Paine does not speak of is the confusion of Mariam's marital state. Non-Jews of the RCC etc applied the Roman or  even modern concept of what 'betrothal ' is to the NT. Among Hebrews it was different. It is not Engagement. In Hebrew and biblical custom betrothal=Qiddushin= consecration. It means marriage vows and a divorce is needed to break it. So Mariam was married and with child.
It was clearly a miracle to everyone because she was an old woman! She was pregnant at what was definitely a post-menopausal age. 'By (the) holy spirit' means by the grace of God as elsewhere in NT.
Luke 1:36 says both Mariam and her cousin conceived in their old age (gera as in geriatrics) and as Sarah, Rebekah, Hannah and other holy women. Isaiah 7:7 speaks of 65 years but it is not clear whether this also refers to a married virgin.
No text speaks of a 14-year old. The 14-year old 'Mary' comes again from paganized demigod theology. The Romans assumed anyone older than 14 would not be a virgin.
There is a further overlay of ignorance or lies relating to what modern churches take a 'virgin' to be and what virgin meant to the Jews and in the Bible etc. The first century Jewish Christians were called Nazarenes Acts 24:5 and continued into later centuries. They were clear on these matters. Gentiles were confused.  Paine did not explore the sources of these errors although some of the scientist-theologians like Newton, Whiston, Priestley, Boyle did.
The scientific method requires that each step is proven before one moves to deducing a conclusion. That's why Paine needs to be read, checking and verifying with the Bible, other authors and Jewish writings. Some of this is summarized in chapter 15 of the free eBook
https://www.academia.edu/25051668/Jesus_James_Joseph_and_the_past_and_future_Temple_Version_N36