I think you are right to not want to attend.
Sorry, for the others posters, but I will be direct.
It will be a scandalous thing.
EnCourage ministry for example is discouraging to attend to a relative same-sex wedding.
I suppose that if the situation is reversed, and the weeding is for a son, this question of not attend will be more evident…
And sorry, but the argument of "your mother had done all for you… " is not a valid one. She had probably not done more than any other mother. We never had to approve our parents.
She is probably not a naive one, if you have never hide your beliefs, she is probably not surprise. So she made choice, and should not had to except to live on her children approovement.
On a personal level, I have ever attend some civil weddings; but refuse to go to a civil heterosexual union of a future in-law.
If my mother-in-law remarry with a man, I will not attend.
We have been on cold for years (not a resaon given, not a word spoken, but I guess she understand and don’t try to harash us, until at least she learned of my first pregnancy).