Indeed, there are many ancient myths about virgin births, as is well-known to students of world religions. (Examples include Attis, Genghis Khan, Horus, Krishna, Perseus, and Romulus.) And yet, as C S Lewis put it, this one is a true myth! It really happened. Further:
The Virgin Birth is prophesied in Isaiah (8th century BC).
Stories of "virgin births" in ancient religions generally come long after the time of Christ, and most of these accounts are nothing of the sort. (Often any atypical birth is compared to parthenogenesis [virgin birth] as if they are equivalent.)
Luke, Paul's traveling companion, wrote 1/4 of the N.T., and adding Matthew's gospel in, we see that 30% of the N.T. books (at least) know of the virgin birth. The fact that it is not mentioned elsewhere is irrelevant; several biblical doctrines are found in only a small number of books.
Jesus was Joseph's son, legally; and, true, Mary wasn't a virgin—after the birth of Jesus (Matthew 1:25). Yet neither fact militates against a virgin birth.
In short, the allegations against the virgin birth of Jesus are unfounded.