I think it is mainly about the variety of why people watch the show, some watch it because it is funny or enjoyable to watch as a pastime. Some watch it because they are jumping the bandwagon from the last popular thing, I watch it because it just gives a smile to my face at how simple and overall kind it is. I'm a regularly negative person, sarcastic and a realist. The show kind of gives me a lighter feeling when I watch it. Plus the community does have it's good people. The common brony people tend to see are the ones who jump the bandwagon or are just immature brats of all ages who can't get over their own egos that people like other things. A lot of the better bronies are more passive. I used to frequent a Brony server, and all day there were just these annoying asshats. But as it got later and later they left and the quieter crowd came in and we all could actually have conversations about the show, but more in depth about the animation, the character ideas and such, not the obsessive brony talk about, "Who ez bestest poneh?". I actually remember hearing a good percent of the military watch MLP, and it can be kinda easy to guess why, they have a stressful job, something like that, being away from families and such wears on a person, they see death, and some have to live with the fact that they possibly just ruined a family, ruined the life of some 5 year old boy who never even heard of America, so a show like that has to be a sort of ease to the mind. Overall, I just like the show, fan made vids are fun to watch due to their creativity, a lot of the music artists are amazing, while the fanfiction side has its main pool of illiterate failures, many can pull off some of the best writing I've seen and the artists are great as well. As for the porn arguments. Get the fuck over it. Porn is porn. Always gonna be there, and if you aren't looking for it, why complain about its presence?
tl dr:
Reasons. Each person has them, the community is basically like Atlantis under the raging whirlpool. Gotta go through the worst to find the best of it.