Mary is not a virgin by the Bible accounts

[SenpaiNoticeMeYouBlindShmuck]

I figured she was supposed to be a virgin until Jesus' birth? Then her and Joseph could go at it all they want.

For most protestants yes, however older branches like Catholics, Coptics and Eastern Orthodox; most of them believe in the "perpetual virginity of Mary". This is mostly because, Catholics moreso than the others, consider marriage to be a sacrament and mostly indissoluble.

It's hard to keep concise, the general idea is that Mary was sinless, so she could only have a child with a lawful spouse (the Holy Spirit); if she ever engaged in intercourse again she'd be commiting "adultery" against the trinity who she conceived Jesus with. For a woman without sin this is impossible, so they conclude she never had sex after giving birth to Jesus. Also as an interesting side note most of them, again Catholics moreso than the others, because they consider the pain of childbirth to be a punishment for sin also insist Mary could not have experienced the pain of childbirth or broke her Hymen, so how Jesus got out of the womb who knows?

There is a problem using the bible to claim that Mary had other children and that Jesus had brothers, because the word it uses (Adelphus and Adelphoi) can be used to describe any male kin as well.