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.
Example 1
Boy 1: The girl just offered me sex.
Boy 2 : Don't sleep with her, she is a Dirty Plate!
Exmaple 2
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?
Dirty platapus is a sexual deviant, usually one of emotional immaturity and sharp witted much like it's animal cousins it's venomous nature can make it dangerous when cornered. Also has natural maddancing skills.
Don't be such a dirty platapus all the time, you don't have to choke them all the time.
Boys who enjoy yaoi (a genre in Japan that contains sexual and/or romantic relations between two men); literally translates to "rotten boy"; corresponding female : fujoshi