The PR (as of Google's last major crawl) is coming from these backlinks:
http://www.google.com/search?sourcei...2Ehtml&num=100
PR5 is pretty good, most adult site hover around the 4-6 area with a select few hitting 7. Looks like you're doing alright but remember that PR is just a guage based on back links. You still need to target the right set of keywords to make full use of the PR you presently have.
WG