One of my closest friends is Christian. I recently recounted my cousin's wedding, which was "offensively Christian" and uncomfortable for me. The sermon claimed that only Christian weddings were valid because people should marry not because they love each other, but because they love Christ and want to have children "in His service." My friend took the comment very personally. How do I explain to her in a civil manner why this is offensive to me, as a non believer?

I would ask her hypothetically how she would feel if at a Jewish wedding she heard that only Jewish weddings were considered valid, or a Muslim wedding, or even an atheist wedding? 

It isn’t even the fact that the wedding was over the top Christian, but rather the fact that the pastor that your friends chose was intentionally exclusive, that he devalued marriages and love of all religions and faith, other than of the Christian faith.

If she still doesn’t understand, the chance is that she isn’t trying to understand or doesn’t want to understand, as frustrating as that may be.