Get your professors to like you, then ask for extra credit. They can give one of 4 tasks: Help at the clinic, the psychology class, the art class, or the library. Helping at the library can eventually lead you to the library's basement, where there's a lot of BDSM equipment.
Continue helping at the library and building up your compliance and/or assertiveness traits. Eventually the librarian talks about having BDSM meetings every Saturday and gives you an invitation if you want to come. I've only triggered this once on a save where my compliance was 4 or 5 and I was someone's sub, but apparently you can also join if you're a dom and you have a sub.