He has the view most men do. The fiancť does not like being cucked. Most men don't and would have been gone .
This is a harsh truth. I'm very non-judgmental. You do you.
I've known many, many people who were in open relationships, claimed to be polyamorous. That's well and good.
They always acted superior to people in a monogamous relationship btw. I always just smiled and nodded.
Out of the many people I know who stated that's who they are, not one of them are happy. All of their marriages ended in a catastrophic and messy divorce... because Humans are territorial. Sooner or later someone (Usually the male when we're talking about a hetero marriage) always ends up very unhappy and resentful and then things blow sky high. I've seen it. I've seen it. I've seen it.
OP: I'm not judging you. I am not. That's not who I am - but you need to step back from him, and you need to truly look deep into yourself and decide if this is 'for real' or if it's a phase. If it's really you, and you come to this conclusion after a LOT of introspection and soul searching, you're going to have to find someone who not only verbally says it's okay, but someone truly okay with it. And that person, in my experience, is a unicorn. That's not him. He sounds deeply resentful and festering about this and it's 'leaking out' if you will.
I don't mean hook up with an actual unicorn... that would be weird. And it is universally known that unicorns are jerks