I feel my words have been taken out of context. Perhaps due to a poor choice of words on my part; eg: the word ‘wrong.’
My point of inquiry was: by telling us Joseph did not know his wife until after the child was born, what message did the author want to communicate to us?
I don’t think the author wanted to communicate the following: sex is wrong, the Catholic Church is wrong to teach Mary was ever virgin, the Protestant Churches are wrong not to believe she was ever virgin.
Another poster stated it was so there would be no doubt as to the Virgin Birth. The significance of the Virgin Birth was Jesus was the Incarnate Word. All Christians agree on this point.
Regarding sex, it was perfectly common for married couples not to have sex. My Grandmother told me in her day, if you did not want any more children your sex life was over as contraception was not readily available. Condoms were not sold in pharmacies and there was no contraceptive pill. Rich Arabs in contemporary society have up ten wives, how often would one wife get to have sex with her husband? Of course he could be very active!!!

I once read of a man in Laos who only had sex with his wife when she had her period as she would have been risking her life having more children. In hunter/gatherer communities, women may not have seen their husbands for weeks on end. The point is we should not see sexual relations between married couples in the first century or ancient civilizations in the same context as contemporary western society. It was therefore not usual for married couples not to have sex.
What the Gospel writer wants readers to believe is Jesus was the Incarnate Word, the Messiah, the Saviour of the World. If the author of the Gospel wanted readers to believe Mary was a Virgin, simply telling them Joseph didn’t have sex with her before Jesus was born is insufficient. What was to stop Mary having sex with Joseph before they married? Are we to believe that kind of thing didn’t go on in Ancient Israel?
In the Gospel account, the miracualous circumstances surrounding the birth of Jesus are recorded before this statement about Joseph not knowing her is made. Was it simply out of reverence for the child she was carrying Joseph refrained from having sex with her? Or is the author trying to tell us something else?
In my personal opinion, Joseph ‘not knowing her’ could have many interpretations; none of which are relevent to the message the author is trying to communicate. But I stress that is a personal opinion and therefore not one is anyone else is obliged to accept.