Why is "slash" (i.e., a homosexual relationship) something that has to be "warned" for, in the same way that you warn for murder, rape, and drug abuse? That's pretty offensive, actually.

If there are going to be descriptions of homosexual activity, it can get the same "sexual situations" warning as any other story. But if I should ever happen to mention a gay character in one of my stories (or, horrors, a gay character who has a boyfriend/girlfriend) does that mean it needs a "slash" warning label on it, even if the story is not really about the gay couple and there are no "sexual situations" described?

Yes, I understand some people don't want any gay characters appearing anywhere in the stories they read. Some people also have other types of characters they don't want to read about, or would not like to read stories with certain religious or political beliefs, but I notice there are no required "warning" tags for those.