Misuse and abuse aren't quite the same. They're similar, in certain contexts, but misuse tends to be more apt in contexts of improperly, but non-maliciously, utilizing a specific inanimate object or part of something else, whereas abuse implies deliberate (usually illegal) malice, for instance sexual assault or domestic abuse.
For example, a kid can misuse his nostrils by putting toy soldiers up there, but he doesn't abuse his nostrils by putting toy soldiers up there. A man can misuse his blender by using it to blend up iPhones, but he doesn't abuse his blender. A gay man can misuse his large intestine by having homosexual sex, but he doesn't abuse his large intestine.
Use of the term "sexual abuse" or "abuse" when talking about homosexual sex implies a very deliberate malice, a violation of the will of either partner, which is distinct from a sexual encounter in which both parties consent.
So yes, consenting gay men may be misusing their anatomies, but they aren't abusing each other by doing so.