I can speak to you first hand about mainstream. I am slowing removing myself from a company that has an affiliate program that was totally mainstream.
www.WinningProfits.com
Basically mainstream companies are very gun shy and despise the adult industry, mostly because of their online success, it is hard to deal with Webmasters and programs.
Now going back to my roots in adult, I along with some great partners are starting a new adult/alternative program with some great products and sites.