This expression dates as far back as man has been stuffing his junk into farm animals. You fuck one goat. Meaning there are some things we do that we must carry with us, no matter the good deeds you do or the people who love you, this can all be overshadowed by one horrific deed.
An old farmer walking through his field with his grandson.
Old farmer- "Billy, you see that fence there? Your father and I built that fence with our own two hands.
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!
Jake: "I regret telling you that. I let Sarah eat my ass one time. You break one plate, you're not plate breaker but you fuck one donkey!? and you're donkey fucker for the rest of your life."