Gateway slut

Hanging out with a gateway slut (who is perhaps a whore or hoe but not actually a slut) will surely result in your eventual acquaintance with her girl friends, many of whom are slutty. Befriending an "efficient" gateway slut will garuntee your hooking up with her friends.

This term is derived from "gateway drug" because of the congruence in starting with something that's not totally awful but eventually degrading your standards to something pitiful.