Unless they are shoe-horned into the actual canon and make no sense in context, then I don't mind these bits of erotic fan fiction at all. The same goes for 'dream fights' for me, as well.
If I felt that adding sexual fetishistic elements to a work were somehow worse than projecting out-of-place political or religious views on characters, then maybe my viewpoint would be different.
That's the kind of thing that should be left to fan fiction or parody. As soon as one goes pro, it is time to let go of match-ups or pairings that would derail storylines just because they would be amusing on some level.
When left to fan fiction or works of parody, sometimes, the more mismatched or outright genre inappropriate the pairing(when dealing with any combination of the so-called 3 Fs), the more hilarious the end result can potentially be.
If the fans aren't petitioning to have any of their extreme fantasies (erotic or otherwise)put into the text, it isn't worth getting upset about.
Then again, I say that as someone who hasn't read a work of amateur fiction in about a decade or so, partly due to having so many professionally written books to catch up on reading. Their synopses on TV Tropes sound like a lot of fun, though.