At least some parts of slash fandom have developed into a really weird direction where this is concerned. Everything even remotely straight is normative and evil, while slash is automatically progressive (even if it's only that one pairing you really want). On the other hand, everyone who doesn't ship a pairing where the partners have different races will likely be called racist on tumblr.

Although even between shippers of het pairings there is some odd "my pairing is less problematic than yours, so it's canon!" thing going on (because healthiness always determines whether you like a couple. And no, they don't really know what canon means. Or Cannon, which is the fandom term typo that I still love by far the most.)

What creeps me slightly out about this is the prescriptive nature and very narrow definition of ship they seem to have. Not that I need people to constantly romanticize icky dynamics, but it's surely healthier if you, say, ship Edward/Bella in a safe, imaginary environment, and get it out of your system that way?