I’m getting some major red flags here.
The first flag for me is that you guys are engaged, but he received a plus one ie the invitation wasn’t addressed to you as a couple. This to me suggests that either the bride and groom are really rude/don’t want you there, or your Fiance hasn’t made people aware of your relationship status/the fact he is in a committed relationship. So that in and of itself would bother me.
The second is how against you going he is. I have several issues with this; first, I cannot imagine myself or my OH ever wanting to attend a wedding solo; not because we’re joined at the hip, but because we are a couple, and weddings are about relationships and love. It seems extremely odd to me to leave out a long-term partner. I also find your FI’s reasons incredibly immature; what is he, 15? You are able to party with your partner there, and it shouldn’t be so ‘wild’ inappropriate that her presence would make things awkward. I woudl also be worried about him having two sides, like another poster said; my OH and I know each other inside out, and we don’t keep certain aspects of ourselves from each other; I would find it odd if my OH felt his behaviour was ‘too wild’ for me to witness
The third is the way he backtracked, after already having gone on about how he wants to go alone because it will get ‘wild’, how you won’t know anyone, etc. He has basically set it up knowing that even if he then says ‘Oh, shit, I didn’t realise that not bringing your fiancee to a wedding was disrespectful, especially when everyone else is bringing their girlfriends/fiancees/wives, and that you would feel hurt being told to stay home! (which, unless he has zero sense is bullshit, of course he knows it’s wrong not to bring you) Of course you can come!’ you won’t then want to come; and that is exactly what has happened, you now don’t want to go ie, he has succeeded and gets to go solo, but with your ‘blessing’ because he gave you the ‘option’ (in your mind; he didn’t, it was set up cleverly with the end goal of him going solo, for whatever reason, in mind)
IMO It’s very ‘fishy’, to say the least. Best case scenario from where I stand is that he is disrespectful, immature and has sides to his personality that you haven’t seen, which would cause me significant concern; worst case scenario is that his friends aren’t aware of your relationship status and he is using this as an opportunity to be ‘single’, possibly to hook up with an old girlfriend/fling. Either way, it isn’t good. If I were giving him the benefit of the doubt and staying with him, I would 100% call his bluff and say that you would love to attend with him, and are looking forward to it; his reaction, and his behaviour at the wedding, may turn out to be quite eye-opening.