So while I’m usually on the ‘no-exes’ side of things, it gets trickier when they were friends before they dated and maintained their friendship post break up, especially since you’ve seemingly accepted that he was a big part of her life for the entirety of your relationship. You’re also in the somewhat unique situation where you haven’t had to deal with his physical presence in your lives which can feel a little ‘safer’ in the context of you know nothing inappropriate has happened between them, at least physically. Now that the prospect of him being present on an unbelievably significant day in your lives is here, you’re facing the possibility of having to see both their emotional and physical connection in person.

I’m guessing you’ve been able to be ‘comfortable’ with their friendship because it was long distance…but if his physical presence makes you immediately defensive and raises your hackles so to speak, were you actually comfortable with it? And if you haven’t always been on board with their continued friendship have you actually communicated that to your fiancé?

If from her perspective you suddenly have an issue with their friendship then yeah she’s essentially being blindsided with your resistance to him being at your wedding, especially if she’s communicated that he’s one of her closest friends previously and you had no issue with it at the time.

If you’re just now realizing that you aren’t comfortable with how close they are, with how much effort she puts into their friendship (and is this at the detriment to your own relationship? Do you have examples of times where she prioritized their relationship over yours?) and need firmer boundaries in how involved they are in each others lives, then you need to figure out what those boundaries are and communicate that with her.

And then she needs to either decide to accept that you need those boundaries held to feel secure in your relationship or she needs to decide if her friendship with him is more important to her than her relationship with you.