That being said, I get a really big amused chuckle out of "Sassy" fans. (The Sam/Cas pairing, for those who don't know). Mainly, just because I absolutely love the portmanteau. Those Sassy fans are so sassy!
Now, the pairings that I think are unbelievable, I can at least see SOME canon bases for them. I mean, the show itself plays up the Wincest angle with the constant gay jokes...or even the more serious moments such as Lisa calling Dean out on the unhealthy co-dependence with his brother...and of course, we get the Dean/Cas thing played up in the show a lot lately...what with Dean and Cas officially having a "more profound bond" or the comment from Dean last year about the way Cas was looking at him...and we got that great belt-biting scene last week between Sam and Cas, so I just know those Sassy fans are going to be running with that for a long time to come.
The one pairing that I just don't get AT ALL (and I see crop up enough that I can rule it somewhat-popular) is Sam/Gabriel (or Gabriel/Sam...whatever floats your boat).
What the hell is the canon bases for that? When Gabriel isn't outright putting Sam through the worst psychological torture possible, he's completely ignoring him in favour of talking with Dean. I mean, yes, Gabriel was putting Sam through psychological torture to try to HELP, but it's still psychological torture (which would definitely not endear Gabriel to Sam) and it didn't really help (which would not endear Sam to Gabriel.)
And you could argue that people write Alistair/Dean stuff too (which I find absolutely disgusting, but you know, whatever floats your boat...) and it's the more non-con, sadistic, thing...which, you know, given the previous Gabriel->Sam psychological torture, COULD be sort of similar...except that whenever I do see the pairing, judging by the summary, it's some schmoopy curtain-fic.
That is a whole lot of words to say: I just don't get it.
I think Sam/Stunt-Demon#2 would be a more believable pairing.
(I should disclose that for the most part I've read every pairing I've come across at least once, if only out of curiosity...because, you know, who hasn't done just about everything once out of curiosity?)
PS: My job is boring.