I think you're the one confused about what virginity means.
It means a woman who has not had intercourse with a man.
A woman born without a hymen or who has had lost it by other means is every bit as much a virgin.
Do you think the "revirgination" surgeries popular for Muslim women actually turns them back into virgins?
BTW, I am a Christian, and I see no reason whatsoever to believe there was anything unusual at all about Mary's delivery or its effect on her body.
If Mary never had physical relations with a man after Jesus was born (which I don't believe, BTW) then she remained ever-virgin regardless of the physical condition of her lady parts.
Mary DID have sexual relations after the birth of Jesus. Matthew 1:24-25 Then Joseph being raised from sleep did as the angel of the Lord had bidden him, and took unto him his wife: 25And knew her not till she had brought forth her firstborn son: and he called his name JESUS.