It's pretty hard for any site these days (especially the big ones) to stop fake profiles and scammers from signing up, but I think I get what you're after.
Once would be fine, but better would be an ongoing casual thing. I'm not a swinger, so I'm not sure about sites like SLS. They may be sold to a different owner who uses different standards or may engage in some of those shady practices to boost membership and use. My first recommendation would be to try Lavalife's Intimate Encounters section; you sign up with them and then decide which of the three sections you want to be a part of (Dating, Relationships or Intimate Encounters).You can use different handles in each section under the same account too, so no one will be any wiser if you want to advertise for one thing in one place without ruining your chances for something more should it come up.The FWB relationship I did have for a few years has moved on and it was lovely for both of us. Married and I've got the ok to wander within safe boundaries and reason. AM will make every effort to separate you from your money, so be careful. Honestly, for the money spent you might be better off getting a 0 Starbucks gift card and hitting on girls waiting for their Latte.Don't respond to IMs from someone you haven't corresponded with already. OKCupid works somewhat in my experience but caters more to poly / open relationships.That said, I've met more girls and better quality off CL than anywhere.AM is Ok, but as mentioned, the competition is ridiculous, women put minimal effort in (empty profile, no pix, sit back and wait for emails), plus it's expensive and the operation is ethically challenged (imagine that! There are problems with fake profiles (either scamming or sometimes guys putting up w4m profiles to check out the competition).If you're going to use it, treat the site like sex with a street hooker - use multiple layers of protection.DO NOT use a real credit card, get a green dot or other reloadable card.