You forget that Unitarians call themselves a "church" and pretend to follow the Bible, even though their beliefs only vaguely resemble Christian beliefs in some ways. And there are some otherwise Christian denominations that have authorized gay marriage.
I never forget the UIni Unis. They are3, however, not Christian, not are they representative of a theos. The only point I am making is that marriage is a rite of sanctification in any traditional religious institution. This has nothing to do with state regulated contracts. I may, if I choose, marry without a license in a church. The church is not required to complete a state license to perform this ritual.