Everlasting Fantasies??

So I've been getting off to 1 thing for a while now, nothing else works for me anymore. I would NEVER EVER try it, but I've been fantasizing about my fiancé having sex with someone else while I watch in secret. The thought is arousing, yes but we both agree that it's not something we want to try. He insists it makes him uncomfortable and I believe him because it makes me uncomfortable as well....but the thought is just so...mm...you know? So my question is, do any of you have fantasies that you wouldn't ever try. Yet they make you feel like waterslide central/tent pitching 101? Please share your stories if you're up for it! Just want to know I'm not alone.