..You fuck one donkey!

The shortened version of "If a man has sex with a man, it doesn't make him gay. But you fuck one donkey, and everyone will call you donkey fucker until you die."

It means that some deeds are so horrifically memorable that you will be known for it, even if the act or event was a one time thing.

Bill: Hey guys! What are you cooking for Thanksgiving?
Simon: I don't want you in the kitchen, you might get a semi.
James: Haha, turkey fucker!
Bill: C'mon man, that was when I was 14. God....You fuck one donkey!