Based on the idea that a pregnant woman has a bun in the oven at the front, a back oven refers to the back passage, usually of a gay man. It can refer to an empty or full back passage.
“Hey Martin, I have just put a batch of Vegan brownies in the Aga. While they cook, would you like to repeatedly and passionately enter my back oven against the kitchen work top?”
“Why yes, Bradley, I would!”
“Why yes, Bradley, I would!”
by Back oven man May 10, 2020