I would have to say that you should state your biases. You were until recently, the friendliest faggot, by your own description. So, it is totally within reason to expect that you would come to the defense of gay marriage.
"I am an asshole, asshole, asshole...."
I have had friends who were gay (not the ceiling hanger, mind you.) I also have not been a "religious" person, regardless of my acknowledging and thanking God for the good things in my life. I can say God bless you and then say something profane in the next breath. I have been a blasphemer. However, god is stronger than that. My sins are more to do with the grief and anguish I have caused others. The disappointment I have been to my mother. (Long story for a different day.) So, if I ever say, "I have been called worse by people closer to me than you," you (in general) will have some context.
I think you are right in that marriage has suffered as a result of these modern actions and often it is about sinful behavior of one person or another.
Marriage used to be about teamwork. My wife's parents were married in 1947, a few years after he came back from WWII. They built a house. My wife was born in 1955. Her younger brother was born in 1958. They were married until death did they part. FIL passed away in 2014, MIL passed away in 2022, at the age of 95. Her family lives to be older than dirt.
And that is how I think marriage should work. My mother was not so lucky. So, I had 3 step-fathers. After the last one, she swore off men. No, not being lesbian. She just dove deeper into her faith as a born-again christian. And I had been to some of those churches with her.
When I was younger, my grandmother and step-grandfather had us baptized as mormons and I was ordained as an aaronic priest when I was 14.
My first wife passed away in 1992. My second and current wife and I met Dec 19, 1993 and we were married in April of 2000. And are still together. I love her like crazy and it will not stop. I am with her until the very last breath.
What's at issue for the OP is the need to vent. His expression of faith is such that he cannot attend the wedding because the couple is homosexual. And I think that is okay. I could say it's okay if he tells them that is why. No need to be disrespectful or insulting. Say I cannot go because I cannot celebrate this and reconcile with my faith in God.
Give the problem to God. No need to justify the faithful decision. God made the rule, let Him deal with it, that's His job.
First off, the "Friendliest Faggot" moniker I was using had nothing to do with my sexuality (yeah, I am straight believe it or not). It was a parody of Milo Yiannopoulos' "Dangerous Faggot" moniker. Second, I'm not the one who removed it. I imagine the staff was getting complaints about it so one day I logged in and see it was changed back to the generic title so I came up with a more fitting one.
I am very much not a fan of turning Christianity into a contest because it is not a competition, it's a path to being a good person and living a good life and following values that have benefited society for ages. I myself have a best friend who is openly gay and is married, but they didn't do it anywhere near a church. Funny thing is if you hung out with them both and you didn't already know they were gay and married, you would never be able to tell. I also have a cousin who was essentially made into a tranny by his single mother at the age of like 8 or 9. I have literally never met the kid and will never have contact with his mother ever again due to her blatant child neglect. I only bring those two subjects up to show that not only do we all live in sin, but we are surrounded by sin as well. God gave us the freewill to decide for ourselves how we deal with the sins around us. The OP of this thread used his freewill and chose not to participate in a sinful wedding. I know it's a hard decision to make when it's friends and family, but at the end of the day, we follow God's teachings to the best of our ability and we don't need any extra sin around us in our already sinful lives.