I'm not religious, but I understand that a wedding is very important in some religions. Catholics for example consider it a sacrament. It's not about their guests, it's about the couple and if religion is important to them they should be able to have that included. You can just not go if you don't want to. It's about supporting them and their journey together. It's not about the attendees being religious.
It'd be like going to a vegetarians wedding and being upset they didn't offer meat dishes. It's their wedding and their views. If there's any day where they should be able to subject people to them (for lack of a better phrase) it's their wedding day.