You can use any of the 'voyeur' tools on this site, and you can also take a look at the Yahoo Top 200. The top keywords are all sexual.
If you're thinking "Hey, if I can get a top listing on any engine for "sex" then I'll get rich." This statement is true, but put it into context: somebody once offered me $20,000 to get their porn site to the top of the list for just a couple of sexual keywords. You're in a cut-throat business.