So am I!
And what I look for in a fanfic is the challenge leading them to be together, which is why I don't mind them going through a relationship with someone else first, as to me it's one possible way to challenge their getting together.
Yes, exactly. An outside relationship that ultimately leads into them being together or even shows how they were together in the past tense is considerably different from an implication that they aren't supposed to be together EVER.
I might not have problems with a story being in context or in character where they're with someone else as a prelude to being together. Or even one where they were together and somehow got separated, by death or whatever. Just how the situation was handled, i.e. the angst quotient, would probably be the deciding factor on whether I could actually force myself to finish reading the story.
THAT, however, is a completely different animal from expecting me to swallow any situation where the other isn't even in the picture romantically, literally or figuratively. Implying that one or both are destined to be with someone else IS out of the context and character of the series. As far as I can tell that's the invisible line for me and a great many other FoLC that's so distinctive about L&C as a series.