No one says that Mary herself was conceived of a virgin. Her parents are Joachim and Anne. I think you are getting the doctirne of the Immaculate Conception (Christ’s redemptive salvation applied outside of time) confused with basic biology.
Jesus was conceived by a virgin. Because the Bible says so. That’s the only one I’m aware of.
It wasn’t biology that confused me, it was what was meant by Mary’s immaculate conception, which I wrongly thought meant was a virgin birth.