You can't propose him to have a clean threesome?
It seems like something that could cool you down from your desires.

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I've tried that. Had a friend over for his birthday. We put on a little show for him that was going well until we wanted to have a threesome. He said to us that having sex would ruin our friendship.

Well one thing I know from experience is that you need to make concrete ground rules and respect them. Because normally it's the fear of bad surprise happening that hurts relationship when a third wheel is in place. lol
I don't know how open your bf is so clearly you have a better judgement on the situation