Natalie,
Couples sex was the only site I knew of that was exactly like that, but most all womens sites have couples sex pics and movies available in the members area.
A good 4-female site would be For The Girls if you havent already checked it out.
Here is a description from their site: "For The Girls offers a complete adult entertainment experience for women. Aside from photos and movies chosen specifically to appeal to a female audience, "
I am sure DallasDeb can tell you more about it, she is one of the owners and a very neat lady to talk to....
http://www.forthegirls.com