Models can certainly be stupid but "Explicit Nudity" certainly does not mean spread legs and masturbation. I used to deal with mainstream models and photographers all the time and "nudity" always means "nudity". Nude girls posing in nature or in front of backdrops is a common subject for photographers, see Femjoy.com or Playboy. In fact if you go to OneModelPlace.com you will find women who WILL do Glamour (Peek-a-boo Nude), Artistic Nude, Nude, Erotic Nude but NOT Adult Nude.
Here is an example:
http://www.onemodelplace.com/member.cfm?ID=64945
You need to add some words that mainstream models understand: "Adult Oriented", "Hardcore", "Hustler-like", "Pornographic".