every niche started somewhere, and I'm gonna take a wild guess and say this niche doesn't sound ultra-specific (it's not like you're copying some other site's Latin-tranny-midget-BDSM-gangbang-with-avant-garde-Korean-electronica-soundtracks formula.) If it's a niche that will sell, odds are someone has talked about it online long ago in a chat room or message board, and surely hundreds or thousands have already fantasized about it.

I'm with Bec, they get the prestige of being the first, but that doesn't give them a monopoly.
Have at it, Lee, and keep us all posted on your successes with it--I'm confident you're going to do a bang-up job and make it a "household" name in ways those other cats couldn't even imagine