There's something on the fringe of MLP fandom that is often used to discredit MLP fandom. It is really something that ought not to be brought up here, but since I think someone will, I'll try to make an oblique reference to it: Rule 34.
For mainstream fans it's really quite simple. Some extremely talented adults made a children's show. People of all ages enjoy and appreciate their work.
If it exists, there is porn of it. If that's the argument against it, you can't have a logo, or icon or image of anything, ever.
ANYTHING. EVER.
Besides rule 34 ponies isn't what is leveled against the fandom. It's the 'fact' that they're all pedophiles. Pedophiles that apparently want to broadcast this fact.
