My 2 cents....
Do a search in Google for the number of title tags with a particular kw or kw phrase by:
title: "big boobies" or [big boobies] or big boobies
"" for in that order anywhere in the title tag
[] for that exactly
nothing for in any order anywhere in title tag
***This is an "assumption" method, but you can assume that since the title tag is so key in SEO, that the searches that return the most results are getting the best results.
For mainstream, since they don't include adult --> What I like to do is use Google Suggest...
http://www.google.com/webhp?complete=1&hl=en
You start typing, and Google "suggests" what keyword/kw phrase you're putting in.
One may assume that particular kw/kw phrase is searched often is Google knows it well enough to suggest it.