Awesome, thought-provoking question. I actually have a bit of an answer. ;)

Back in the day, there was a sect or group of Jews who dedicated themselves to God, wholey and completely, similar to our religious orders. Part of that pact with God was perpetual virginity. Convents and Monastaries were not yet established; the people got married but never committed themselves to their spouse in a marital way. Remember, at that time it was shaming to the family and the people who did not have a marital partner, so they got married, but vowed to each other and to God to keep their virginity.

It is believed that Mary was of the same sect. She was dedicated in the temple by her parents, as was John the Baptist.

Now, I received this info from a public tv program, a more secular venue. Never did check it out for validity, but it makes sense to me, because I'm thinking, that's why the priests, even when they were married, were discouraged from being with their wives. Everything the church does is based on tradition, so the tradition of virginity had to happen somewhere along the line for the church to accept it and promote it. Of course, after a time, the church even proclaimed the priests should not be married to a woman, but only to the church so as not to be divided in any fashion from serving God through the people of the faith in their care.

Also remember, that in the ancient language there was no word for cousin. Everyone was brother and sister; including the people in the community, the neighbors and friends. Anyone who proclaimed God to be the true God was a brother and sister. When Jesus was told your mother and your sisters and brothers are waiting for you outside, tradition tells us that it was in fact family, but not so closely related as brother and sister. They could have been close childhood friends who worshipped together in the temple.