Personally, I would go.
I’m a bridesmaid in a Protestant wedding in a few months. I wouldn’t have told my friend to go scratch simply because she doesn’t believe in transubstantiation. I am going because I love her, she’s a dear friend who has made me laugh when I’ve cried. She’s sat there with me and a pint of Ben and Jerry’s and broken hearts. She’s given me rides home when I’ve been FAR toooooo drunk to drive. She has gotten me drunk and she has helped me sober up, she’s held my hair when I was puking. She’s sat in the emergency room with me when I thought I had appedicitis, and was 200 miles from home. She calls me when she thinks of me, usually when a celebrity gets arrested or gets engaged, and she has the best dirty joke collection I have EVER heard. I have done and tried to do all the same for her, except my collection is not dirty jokes, but fabulous shoes.
I’m going to support her, and her choice to marry a pretty cool guy who plays a mean game of beer cup, and who also has a good joke collection but who also has been wonderful for her.
Weddings are about going and supporting your family whether they are by blood or choice, in taking a bold step into the future with someone they love. Saying that you stand by their choice and you will always be there for them,
Also there is usally an open bar with expensive champagne and other sorts of free booze. This reformed party girl still can’t turn down free alcohol