I have to disagree with your basic plot line for most slash fiction. Well, the friend and random pursuing character parts, anyway.
The idea of Character X and Character Y meeting is pretty consistent with most, if not ALL, romance stories, even those published. So, that part is almost a moot point.
I do agree that most often you see a friend character, but it usually ends up with the friend being close to Character X, the view being misinterpreted by Character Y, drama ensues, sex happens, the misunderstanding is lifted, life is happy.
Of course, the general plots used to get two male(possibly female; I don't read much fem-slash) characters together are going to be generic since nearly all the ways for two people to get together have already been written about. It also doesn't help that most(not all) of slash writers are screaming yaoi fan girls that get their ideas from yaoi manga/anime.
I have to admit that I was one of them, so I have a sense of why this shows up a lot. Well, I actually still am, but I write better than that, now-a-days.