I stayed home from a male to female wedding I didn’t approve of. I have never been invited to a same sex wedding although I have nieces claiming to be lesbians and a nephew claiming to be gay so it could happen. I don’t have any idea what I will do if it does happen.
I suggest that you should decide NOW what you plan to do. No need to tell anyone else your plans, but failure to plan is a plan for failure.
Fortunately I haven’t had to deal with that stuff much. Besides my parents, I can only think of a small number of good friends who didn’t remain married. Only one got remarried and it was after we lost touch.
I did have one skanky cousin who called to tell me that she left her husband and was getting married again (to the jerk who knocked her up)
I told her the batteries in the clock I gave her for her first wedding still worked so consider that her gift.
It happened to me and I didn’t go. God’s way or man’s way. I chose the former.
I'm generally very big on etiquette and think we need to defer to these established societal rules in general. But that etiquette needs to be buried.