A presumably platonic outing with a friend's spouse or significant other, usually, though not always, where a certain level of unstated attraction is involved.
Comes from the scene in Pulp Fiction where Jules eats the Big Kahuna Burger and says, "my girlfriend is a vegetarian, so that pretty much makes me one too."
I came home one day and my wife said we were vegan. It's a lot easier to go along with it than argue with her, so I'm a Pulp Fiction Vegetarian.