Say, what do you think about forums like Shochan.org and Flatchan.org? They're lolicon and shotacon forums where people share and discuss a certain genres of anime and manga that show young children in sexual themes/situations. Both forums (like Renchan before it went offline) grow huge fast. Shochan had at least a couple thousand members after a few months of being open. I was even thinking of starting a site for such, as I'm mildly into shotacon. It's legal and illegal in certain countries, so that's something to think about when buying hosting.
Well, in the U.S. there is freedom of speech, and people love controversy, so if you are creating a forum purely to get members, it is a good forum to start.
To be honest, I would say, not matter how controversial the subject matter, as long as you are doing it in a legal fashion and it's something you want to make a forum about, go for it.