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.