Not really my thing

My SO has a kink. He loves fantasizing about me fucking other guys, and constantly asks me to find guys to do stuff with and find a girl for a three some. The thing is, we have tried it before, and it ended terribly for me. He just keeps begging and begging for it and doesn't seem to care if I'm uncomfortable with it. I don't know how to approach the situation, because I do like the idea, but I'm not comfortable doing it yet.