Around here, "normal" is kind of the N-word. The important thing is do both partners agree that an activity is okay (barring physical abuse and criminal things, etc.). Clearly this new avenues causes you distress, and that is reason enough to make the activity off limits. But the larger question is what's up with him that he's thinking along these lines?
I'm guessing he's looking for more stimulation by playing with thoughts of more out of the ordinary stuff. He may have had these fantasies for years, but until you started the communication exercises, he didn't feel comfortable bringing them out. (The IDEA of having multiple partners is a turn-on for me, but I would never do it in real life in a million years. I've also never had a partner who really wanted to know what my fantasies are, and I have some pretty elaborate ones!) I think it's important that you accept the fantasy and not run screaming from the room (even though you may want to), but make it clear that playing with the idea doesn't mean you are going to do it. Is he just sharing fantasies, or does he really want to do this? If you two work with the fantasy, will that work for him?
I'm sure others will chime in with their ideas. This is a really smart group. Just my 2 cents.
