The worst thing you can tell a friend is “Please don’t sleep with [so and so]. I like him and it would really hurt my feelings.”
This kind of admonishment tends to backfire, especially in the cutthroat gay-bar world. Telling a party pal that a person is off limits will only make that person extra desirable! It will put the demon seed in the head of your friend, who will surely mutter, “Hmm. I never thought of [so and so] sexually before, but sudddenly it seems extremely appealing. If HE’S so into him, the guy MUST be someone worth mounting–pronto!”
They will assuredly hook up, especially since, after a few drinks, your friend will only remember something about “so and so” and “sex,” forgetting that it was a “Please don’t” kind of proposition.
So just shut your trap and let it play out in the sharkpool, hoping that you never find out what might hurt you.