Yes, that's the rub: if they're already inclined toward the same sex, it's not really slash, which by its nature is about changing canon/adding in subtext to make straight characters fall for each other. But it's a bit fuzzy. As you say, we look for our m/m under the category of slash even if the characters are already gay or bi, so now there's grey area for sure.

I think the issue of chan comes up mostly in HP where the canonical characters for many of the books are underage, and authors definitely learned to warn for it. But some people love it.

I need to talk with some True Blood afficianados about any homoerotic content that could probably be traced to producers knowing about women's desire to see that on screen due to the widespread emergence of slash. It's conjecture, but probably accurate.