THE EARLIEST OF THE FOUR GOSPELS makes no reference to the virgin birth, and neither does Paul, who wrote earlier still. On later evidence, however, many Christians have made it an article of faith that it was the Holy Spirit rather than Joseph who got Mary pregnant. If you believe God was somehow in Christ, it shouldn't make much difference to you how he got there. If you don't believe, it should make less difference still. In either case, life is complicated enough without confusing theology and gynecology.
In one sense anyway, the doctrine of the virgin birth is demonstrably true. Whereas the villains of history can always be seen as the products of heredity and environment, the saints always seem to arrive under their own steam. Evil evolves. Holiness happens.