Naomi24 hit the nail on the head earlier.
The reason that Jesus, the Son of God, was born of a virgin is so that the oral tradition of virgin births can be maintained. Why? I don't know, but it seems that if you're going to tell tall tales of earthly Gods, it's better (and presumably more credible[!]) if those individuals are born of virgins...
The Greek demi-god, Perseus was born when the God, Jupiter visited the virgin Danae;
Buddha was born through on opening in his mother's flank;
Krishna was born of Devaki - by "mental transmission";
Horus was born of the virgin, Isis;
Mercury was born of the virgin, Mia;
Romulus was born of the virgin, Rhea Silvia.
… the list goes on and on.