Also as a non-christian: I think you’ve got it backwards. Christians believe that she was a virgin because they believe Jesus was their god. Basically that it doesn’t make sense to them that a perfect being could be gestated inside a dirty sex having woman, so it must be that she was a virgin.

Also Jesus had a habit of giving people nicknames (eg Simon Peter - the rock) I could see him having a disciple Mary the virgin and another one Mary the mother that got conflated. There’s no evidence for this but idk it still seems more likely than virgin birth.