Maybe one of us is overthinking things but I never had “legally binding” as one of the requirements for a ceremony to be a wedding… what do you call a non-legally binding non-wedding?
I don’t know of any exes of mine who’ve gotten married; I’ve attended several weddings where at some point there had been interest going on between the guy and me, but always invited either as “family” or as “part of the gang”. If the bad ex invited me, I would be too busy laughing to go; if the good ex got married I’d love to be there - but he probably wouldn’t want me to, he got horribly nervous when he was dating (and later living) with an old common friend and he’d see us together. I wasn’t going to give her any information on him that she didn’t already have!
A Handfasting. Scroll down for Modern useage, neopagan edition.
ETA: When you’re part of a community that does both legal and non-legal marriages, the distinction is sometimes important to people. Seeing as he and I were still legally married at the time (in the process of getting a divorce) it was an important one to the State, in our case, to prevent a legal charge of bigamy. (And an additional reason why tongues would wag if I was there, I’m sure.)
Considering the length of your OP, I would gather that you don’t have too many feelings about this issue. Based upon that, I would pass on going to the wedding, unless you enjoy weddings and you want to go purely for that aspect.