The First Amendment is part of civil law, though. So marriage is recognized as part of “free exercise thereof” of religion and a right that no government can abridge. Not so homosexual marriage, which attacks the marriage institution/sacrament of religion.
I don’t see how that invalidates anything I wrote. There still has to be a legal definition of marriage within a state to have laws about marriage within that state, regardless of what people do as a matter of religion independent of state law.