I think its likely the bride. But also that whilst you may view him as a good friend, even a best friend, perhaps he doesn't view you in that way. Either way if his wife to be (possibly) thinks the 2 of you are too close and doesn't want you at the wedding itself, then not much you can do about it unfortunately. I doubt he will argue that you should be there, he will put her 1st
I suspect your friendship will drift after his marriage anyway. Perhaps he talks about you a lot and her nose is out of joint/she's fed up of it
& for all you know he could have had the "yeah she fancies/fancied me" the big "I Am" chat with her, in the past. I don't put it past men to do that kind of thing to make themselves appear sought after, in a way. Then when gfriend doesn't want a woman around who has her sights on her man, gfriend gets the blame and said man gets away with it without having to explain himself as you are just phased out
Or even that he assumed you did fancy him, since he was your +1 at weddings etc, as opposed to say you inviting your own partner, or a woman friend at times
In your shoes Id decline very politely with a genuine sounding excuse, then have a nice day to myself doing something nice/pampering, and not focus or the wedding or snub.
Its different times for you and he both now, marriage changes the dynamics may not be entirely fair but I guess we often find life is that way, not fair at all at times