Specifically, over the fact that it's illegal for ministers to officiate wedding ceremonies without a valid marriage license, which basically forces them to either break the law or discriminate against gay couples. They claim it violates their First Amendment rights to freely practice their religion and frankly, I don't see how it doesn't. They're being legally prevented from marrying people in the church, which has been important to Christian couples in same-sex relationships even if the state doesn't recognize their relationships.