Yes, it is true that the ancients had limited knowledge on the mechanics of conception; but why the emphasis on virgin births in myth and legend if not for the knowledge that to fall pregnant one must have sex?
Virgin births have been rampant throughout the myths and legends of humanity as miraculous phenomena – the oldest I can think of is the virgin birth of Horus, member of the ancient Egyptian pantheon. Then there is Greek/Roman, Persian, Hindu, Buddhist, Hebrew, and even Aztec mythology. Cultures extending from the Mediterranean, to the Far East, to the South Americas – all isolated from one another for thousands of years – telling stories of immaculate conceptions.
Then there is six thousand year-old Sumerian creationist poetry depicting the necessity of the creator’s “seed” to bring forth life from the virgin earth (the earth personified as feminine). So there must have been some acknowledgement that ejaculation had something to do with it.
Such accounts were made so profound within these cultures because of how unbelievable it was for a virgin to be with child. The consequences of sex had to have been apparent to them to acknowledge that virgins don’t give birth (without extraordinary intervention); otherwise, the Virgin Mary story would have been met with a casual “so what?”.