Boyfriend likes me to have sex with other men in front of him

So my bf has this fantasy about me being with other men while he is watching and maybe joining in. He is really turned on by that thought. His fantasy didn’t come true yet and I am not sure if I want to make it happen. I am also turned on by that thought but for me it’s good enough as a fantasy, I don’t need to make it happen in reality. Or at least I don’t think so. I am just scared of it to become something regular, I don’t think I want that. Why are some men so turned on by it though? I am not sure what to do. I would appreciate if someone wants to share their experience and also men share why they like to share their wf/gf?