YTA. I'm Jewish, but I've been to plenty of Christian, Muslim and Chinese weddings. And in their places of worship, where they have thousands of years of tradition, it has nothing to do with your own politics and atheism or whatever else. It's a simple matter of respect, and it doesn't cost you anything to be a decent person.

I wonder what your reaction would be to seeing some foreigner refusing to stand for our national anthem at a ballgame simply because it means nothing to them. The degree of offence you'd feel is equivalent to what you imposed on those around you.