It's a shipping thing to be honest.

There's tendency to villainze the SO of any character we ship with another character. I saw it happen SO SO MUCH to Mai by Zutara shippers. It was depressing.

I think with slash, it just comes across as more misogynistic because some people seem to think slashers are saying NO WOMAN is good enough for the characters, rather than just hating on the SO because she gets in the way.