Wouldn’t a Christian say there is no such thing as a same sex marriage? He could/would attend a same sex civil ceremony for relative or close friend. But there is no such thing as a marriage between two same sex persons. They want to call it that but it is not.
My gay friends and relatives (step relatives) would not be surprised at me not participating in their wedding, or civil wedding, and I would not have any problem explaining it to them.
When it doesn’t involve “gay”, people don’t have a problem with this kind of personal values thing. Many parents love their children’s boyfriend or girlfriend and know that they live together, and may even visit their apartment, but the kids aren’t shocked when the parents don’t allow them to share a bed in the parent’s home, it is just accepted as a personal boundary internal to the parent.
If your anti-war sister refuses to attend your son’s graduation from basic training, does everyone freak out? No, they accept it as part of her values boundaries and they get on with life and she shows up for dinner later in the day and everything is just fine.