Most bronies are not furries, they are just boring contrarians doing the trollface.jpg in the hopes that they will piss off some random dude on the internet who doesn't like bronies. The reason why PonyGAF and /b/tard bronies exist is because there are a lot of socially marginalized people on the internet and to be a part of any group, even if it towards My Little Pony, is worth the pursuit. They're the duders who are on the low end of the totem pole of nerd hierarchies, which nerdery is in and of itself on the bottom end of life's totem pole. It's a sense of community, even if it is over cartoon ponies meant for little girls. That in your face contrarianism is a way for losers on the internet to feel cool, if only for a little while, before they log off and work the third shift at GameStop making $8.25 an hour well into their 30s.
I used to get worked up over them but then I just ignored them. With AdBlock and ignore features, as well as simple willpower, you can remove 99%+ of brony tardery on the internet.