I never really thought about it, but when it comes to a lot of fandom I'm always a far bigger non-canon (slash/yaoi/shounen-ai/whatever mainly supporter) and I ponder to myself is that friggin normal, especially some cases.
Some fandoms (with examples) >.> and for some of the freakier ones my "reasoning", which doesn't have a reason at all besides my being depraved:
MR: Ari/Fang, Fang/Iggy.
Inheritance Cycle: Eragon/Murtagh... now bear with me I know some people might think, "OMG they're half brothers that's sick, which it may or may not be, but I see them having an intense relationship and Eragon at least doesn't seem to look at Murtagh as a brother. Also I never really like Arya (sp?) and I hate OC's so nowhere going for that.
Naruto: Naruto/Gaara, Kiba/Naruto, Sasuke/Itachi (this could pretty much be summed up with part of the above explanation).
I'd add more, but the list could be endless. So lets go to canon/implied-possible couples I like/tolerate/meh.
MR: none really so far... has there been anything I'm truly missing?
Twilight: The rest of the Cullens' relationships.
HP: Remus/Tonks, Hermione/Ron, Bill/Fleur, ect
Inheritance Cycle: N/a
Naruto: Asuma/kurenai, Shikamaru/Temari > they go so well together.