Some of the narrative in the gospels is concerned with fulfilling the prophesies of Isaiah, since that would build a case for Jesus being the Messiah, which was something people were interested in at the time.

Isaiah didn't clearly prophesy a virgin birth, but the writers of Matthew and Luke were probably working from the Septuagint, a greek [mis-]translation which turns the prophecy into one of a virgin birth. So, thinking that a virgin birth was required, they wrote it into the story.

The question of whether someone claiming to be made pregnant by God is making a plausible claim, is fairly moot when the whole episode was probably made up anyway.