To be brutally honest and like some of you folks here already said, anti-bronies hate people who fall outside of the "norm", and instead of understanding it, they wanna make us go away. When you ask them why, they will just give you a bunch of witty insults like "BECUZ U'RE GAY", "YOU'RE AN AUTISTIC FAGGOT" etc, but deep down under the hood they are so adhered to the gender stereotypes that they believe that anything outside of their "norm" should be annihilated. Just try to go to Youtube and search up news report about bronies, and take a look at the comments section, and you'll know what I mean.
I once got flamed by an anti-brony on deviantART, and after a couple of minutes of browsing his profile I found that this guy is really a rabid hater, making a LITERALLY 9000 characters of rambling how ponies suck and how his day is utterly ruined and his eyes raped by a ponified Solid Snake on his Google image result. I tried to argue with him but he said that I'm just going circles with "stereotypical brony shit", even I try to be rational and admit that our fandom has flaws he just replied "That means your fandom sucks lol. Period."
It was then I realized that arguing with this kind of people is like arguing with a racist. Basically if you're a race they don't like they will just tell you to fuck off and won't listen.