Apologies for bringing this thread back from the dead but I'm new to the Nexus. I saw this thread and felt like I should share a great resource.
There's a really good book out there (The Ethical Slut) that I like to recommend to people when this issue comes up. The phrase is mentioned earlier in this thread, but it's an actual book, too. Its focus is primarily for the non-monogamous but as a whole I think it has a lot to offer the "plainly promiscuous," for lack of a better term. The authors reclaim the term slut as "a person of any gender who has the courage to lead life according to the radical proposition that sex is nice and pleasureis good for you" (an idea that I've found myself in agreement with for most of my adolescent-adult life, FWIW).
We atheists have our own individual morals and for me, what it boils down to is this: if you enjoy being promiscuous and you are having good experiences with it, then there's absolutely no reason you should feel bad about it. Personally, the only time I can see it being a bad thing is when it is being done to deliberately hurt people or relationships.
I grew up in a very sexually repressed environment, so my own struggle was not with my opinion about the matter, but with hiding it (and finding others that shared my opinion). This book helped me in a lot of ways, and I hope it will help you as well.