Is slash helping to reduce heteronormativity in real life? For me, I can answer with a definite yes. Simply because they showed a broader amount of relationships, and sexual orientation besides heterosexuality. Reading enough of those stories changed my automatic assumption about people's sexuality, including a good deal of clichés that came out of movies/TV shows.
I am, however, not a rabid slash fan, but more a reader of het and slash, and even some gen, if I like the story. I'm not in anime/manga fandom, so I'm really not acquainted with yaoi fanbrats.
I guess some of the struggles between het- and slash-fen may come from a time where slash was hidden and looked at strangely. So pehaps slashers feel attacked by people not liking slash, and het-fans don't understand slashers feeling attacked. That's just some speculation...