Fiancé wants open relationship

I've been with my partner for five years, our wedding date is about a month from now. We are both really excellent communicators and love each other a lot. Early on in our relationship (like, day one) we discussed what we were 'looking for' in a relationship. At the time of our meeting we were both looking for something casual, we were dating other people. 

My partner has always identified as somewhere between monogamous and poly, and I'm personally very monogamous (I've done the deconstructing and I really don't feel that my lean towards monogamy is repressed or tied into internalized patriarchy or whatever, I really have just always wanted one partner to do life with). 

My partner was the one to close our relationship/suggest we become exclusive. Because I knew my partner was a bit more flexible between mono/poly I checked in regularly with him throughout our relationship around whether he still felt content in a monogamous relationship. He always said yes, and eventually proposed. We've been so happy planning for the wedding over the past year and a half.

Out of the blue about two days ago he shared that he'd done some self reflection and that he needed an open marriage to be happy. I was shocked, hurt and confused, and I feel like an idiot because from an outside perspective I know exactly what this looks like and my first instinct (as someone who has a core wound around never feeling like 'enough') was to run/shut down the relationship.

He's my best friend and I truly believe he loves me, and he said he doesn't necessarily want to date other people individually but that he might want to include more people in our relationship. I'm initially open to the idea of occasionally including more people in our relationship in a casual way, but I am not and know I'll never be comfortable with the idea of him dating other people/having a separate relationship. He says he's okay with that, but I feel like I don't know how to trust him anymore if he could spring this on me after years of being happy in our monogamous relationship.

Any advice from anyone who has gone through something similar is greatly appreciated, I'm overwhelmed.