It seems every time I step into the world of a slash fanfic, I'm confronted with a reality in which females (a) no longer exist, or (b) are now bitches, cheaters, and rapists. I'm not sure which is worse.
Recall that, in slash fanfiction, the author takes two characters – often with female love interests – and makes them gay together. For example, let's take Harry Potter fanfiction. In the books, Harry and Ginny are together. When writing a slash fic, the slash fan handles this in one of three ways:
(1) Harry's relationship with Ginny is ignored.
(2) Harry's relationship with Ginny isn't “real” - they aren't really in love, they just think they are.
(3) Ginny is an evil, whorish bitch.
The first two are acceptable. The third is depressingly common. Through slash fanfiction, we are taught that female characters should be written out of the story – always off-screen, if mentioned at all – or used as villains.
Slashers: I love you. I'm one of you. But most of you are female, which makes it horrifying that you'll repeatedly bash your own gender. Trust me: we get enough of this from the rest of the world. We deal with rape and misogyny and harassment, and when we complain we're told to stop whining. I don't care if you think that Ginny – or any other female love interest who gets in your way – is a Mary Sue, or a whiny bitch, or a two-dimensional character. Treat her with the same respect you treat male characters.
If you need to break up her canon relationship with your favorite MC, then do it gently. It's okay to have them still be friends. Sometimes, couples just don't work it out. Say they had a disagreement and broke up, and leave it to that.
(Or make her a lesbian. I'm willing to deal with that. Hell, it's my favorite trope.)