A dirty plate is someone who has slept with a lot of people and picked up a few diseases along the way. A slut is a plate, because they offer sex to you on a plate, so a dirty plate is a slut with STDs.
Boy 1: The girl just offered me sex.
Boy 2 : Don't sleep with her, she is a Dirty Plate!
Friend 1: Why do all the guys sleep with Hannah?
Friend 2: When sex is offered to a guy on a plate, he is going to take it!
Friend 3 : Yeah but wouldn't you want a clean plate?