@K, that’s really not fair. Yes we all have fantasies, and they’re perfectly normal and healthy to have. Sex isn’t a one fits all scenario. What one ex/FWB brings to the table is not the same (nor is it necessarily better or worse) than what your current partner is bringing.
Anyway, LW. I suggest tell your fiance that while you may have a crush (on someone you haven’t met, but have formed an opinion on), you don’t want to entertain the 3-some idea with him involved. If you’re fine with a 3-some and discussing it, fine, but it doesn’t need to involve that other guy. You don’t even know if you’ll like him. You’re working with the idea of him. When you meet him, you may not even be remotely interested. Who knows. Anyways, you and your fiance need to take time to discuss all of this, and whether you two want to experiment beyond just keeping it fantasy.