I used Couchsurfing a lot in 2010 and 2011, while I was doing research for my book. I never had a bad experience or felt uncomfortable. (Though I was choosing people who actually had a spare bed for you to use, rather than a couch, and I wouldn't be surprised if that kind of filters out some of the sketchier arrangements.) I have a friend who hosts a whole bunch here in Minneapolis, and she's never talked about any bad experiences. I have another female friend who has used it to travel and has, in the course of those travels, had sex with a few hosts. But that wasn't anything she felt uncomfortable about.
Basically, I'm sure there are some creepers out there. Heck, I've read profiles where I was like, "LOL CREEPER". I am also certain that people have had negative experiences and felt pressured or unsafe, because, frankly, that happens in all kinds of situations. But based on my own experience and the experiences of people I know, I'm just not convinced this is a huge problem with Couchsurfing.
(All of this said, AirBnB has kind of replaced Couchsurfing for me at this point. Not because there's anything wrong with Couchsurfing. More just because most of my travel is work-related and after a day of being socially "on" for work I kind of like being able to come back to space where I can turn the switch off and relax, and I can't really do that if I'm staying with people I don't know and whom I'm not paying. Being entertaining and friendly and helpful is the payment, there, and that gets exhausting on work trips. Sometimes, it's definitely worth it. I'd do it again if I were doing reporting that needed to get the flavor of a town. But most of my recent trips, I've preferred to pay for my lodging in money rather than in social interaction.)