This is very much a reason to get cold feet.

Your fiancé realized something about himself and feels the need to explore that. That’s fine in and of itself. But that also means that he’s not ready to “settle down” and commit to being in a monogamous relationship and marriage to you. That is also okay. But that is the reality that the two of you need to accept.

You know you’re not interested in a form of open relationship where he’d be able to explore his sexuality physically with men while also committed to you. That’s fine. The two of you need to have an honest discussion about what you both want and need for your lives and from each other.

If you come to the conclusion that he needs to explore this and he can’t be happy and healthy without doing that, and you can’t be happy and healthy in a relationship where he does, then that just means it’s not the right timing for you two. And that can be okay. Sometimes we meet great people but at the wrong time in our lives and that just is what it is.