1) if you're just friends, then it's completely out of order. You can't just invade someone's space. If they're letting you hug them, it should be a friendly hug, which it isn't if you touch them in a sexual area.
2) even if it's your girlfriend, she's probably not looking for something sexual if you're just giving her a hug. This depends on the girl, some won't mind, but it can feel like you only want her for sex, because you're bringing sexuality into what should be a normal non-sexual thing.
because it's sexual and therefore inappropriate if you're not going out. because when you see regular people hugging on the street who aren't couples, they don't touch each others' asses. therefore, because it's socially unacceptable.