I don't think casual sex has been a major use of Facebook for a while- certainly during it's college-only days that could have been the case, but less so in a more social network aware world.

I think something like BangWithFriends is doomed to fail simply because of the visibility. People can see if you use the app, which would embarrass many. And like almost every single online dating/sexual site out there, the number of men dramatically outnumbers the number of women.