If you want to do more than just meet people online, very open minded, and you're over 18, you may want to try out Red Light Center. Center is a very adult virtual world. You can download and use Red Light Center for free if you just want to meet other open minded people.
If you want more than what the free version of Red Light Center offers then you'll need to come up with $20 per month. The VIP membership lets you get naked and get down. You can't do that with the free version. You'll also have access to the VIP only clubs found around Red Light Center.
Although I like the idea of an adult only virtual social networking site, I'm not quite sure why anyone would pay the rather large amount of $20 per month when you can do all the same sorts of things on Second Life for free. That said, I did join and look around Red Light Center and found that, for me at least, it works better than Second Life, at least my avatar is able to get around faster.
Unlike Second Life, Red Light Center is a completely adult only community. You won't have to worry about upsetting anyone because you're all there for the same reason. There are all kinds of adult things to do, most I can't mention here. If you are an open minded adult, who isn't shy about sexuality, you just may like Red Light Center.