I think what's happening there is that some slashers (because, to be fair, not everyone is like that... only, apparently, the most vocal ones...) mix up fandom and real life. Heteronormativity is still the prevailing concept in real life, so some apparently aren't able to distinguish the privilege heteronormativity has in real life from the privilege slashers have in fandom. Their simple equation seems to be "LGBT people are non-privileged IRL = slash is non-privileged in fandom".

Basically, all that privilege crap and the labeling sucks mighty lemons. It usually ends up with people not wanting to be a majority, because majority equals privilege equals not having the right to criticize (hence, some slashers not wanting to see that they are the majority in fandom). It's honestly the worst approach to discussion I ever encountered.