There are two points in particular that bear repeating.
And the slash I have seen [because I had to have seen it to have developed my strong distaste for it] is mostly misogynist. There's very little involving getting the two boys together without slamming the girl[s] one [or both] of them was canonically involved with.
I've noticed this a lot. Hell, I've noticed it with favorite heterosexual couples. And this is not limited to fanfic communities (Hell, I've probably been guilty of it myself).
I've seen male characters trashed over it too (I think I've directed some insults at Roy Harper in the past), except they don't tend to get gendered insults and slut-shaming like female characters do. And when you go with slash, which tends to be male-male pairing, you have the danger of the only female character in the whole story being made unrecognizable because she interferes with the preferred male-male pair and needs to be taken out of the way.
It's entirely possible that this sort of mindset affects the writers, and explains some out of character behavior we've seen. Could Ben Raab be a Donna/Kyle fan? I'm pretty sure that Dave Gibbons preferred Kyle/Jennie.
Point Two (which cannot be said enough):
Writing/drawing slash does not make a slasher a homosexual rights activist. Wearing your rainbows, going to the rallies, lobbying the government is what makes an activist.And judging by some of the yaoi and slash debates I've seen through When Fangirls Attack, writing slash doesn't necessarily make you the slightest bit sensitive to homosexual issues.