@TARINunit9: the thing is, our fandom is currently under a lot more scrutiny than most other fandoms that have a flankload of R34 content associated with it, so we have a double standard to worry about. No-one looks at someone who says their a Transformers fan and says "Oh, so you're into this?" before pulling up an R34 image of Megatron buttsecksing Optimus Prime. It's accepted that Transformers is a normal thing for guys to like, so anything perverted relating to it is regarded as fringe.

With Bronies though, we are still relatively few in number compared to other fandoms, and since we like a cartoon that is still considered by many to only be appropriate for little girls, when they see R34 pony content all over DeviantArt many of them jump to the conclusion that when no-one's watching we're all trading around bestiality porn, or that we want to molest little girls through the show and toys.

Being adult male fans of a "little girls' cartoon," we're in largely unfriendly territory, and if we do not present the best public image that we can, people are going to keep slamming us with all manner of unpleasant stereotypes. The biggest difference between the adult MLP and Transformers fandoms is that the uninformed public did not have a negative opinion of TF fans by default.

The problem with R34 pony content is that it makes ALL of us look bad, and our fan community can't really afford to have image problems involving pornography since we're under enough scrutiny as it is. As a religious person I know how easily a few bad apples can spoil the barrel.