A Bachelor Plate actually refers to a lack of an actual plate. Most of the time the use of a bachelor plate entails eating over the sink or a trash can in order to avoid cleaning/owning an actual plate.
Man 1: Dude, your steak is ready.
Man 2: Why did you just bring me my steak on a fork? Where is the plate bro?
Man 1: I don’t own any, I use bachelor plates, so you can either eat over the trashcan or the sink, your call.
Man 2: Sink, I call the sink!
Using the inside cardboard of a pizza box or pizza box lid for 'a plate' on which to eat other than pizza. There may be slight grease or cheese residue on the box or lid, but this does not deter the bachelor from using either flat surface as a plate. Using a bachelor plate means that there are no clean dishes, every dish is dirty and while it's a food poisoning risk, the bachelor is too lazy to wash dishes.
This is sometimes referred to a Dorm Plate as well, since college students don't keep dishes in their dorm rooms and dorm kitchenettes aren't stocked with much except fraternity emblazoned mugs. Pizza boxes then become employed as Plates.
One dude to another: I've got 2 plates left.
Another: Whaddya mean, I don't see any frickin' plates.
Dude: (Opens used pizza box) One here (points to lid) and one here (points to bottom of box)
Another: Man I'll eat off my car hood before I eat off that. Nasty!
Dude: What's wrong with my bachelor plate ??? I eat off these all the time. Haven't gotten sick yet.