These laws vary from state to state. Check out this little controversy in Ohio:
The domestic violence statute covers assaults against "a person living as a spouse," and thus recognizes a relation of unmarried couples which approximates marriage.There is more discussion here.
So now Ohio same-sex marriage advocates are arguing that since gay couples are not allowed to have relationships that approximate marriage, then they can beat each other up and the penalty will just be a misdemeanor. If only they could marry each other, then they could convict each other of felonies!
This is really wacky. The problem here is not same-sex marriage, but the radical domestic violence lobby that has over-criminalized minor marital spats. The law should protect us from stranger attacks, not from whom we decide to marry.