Hence my belief that Gov't needs to exit the business of marriage altogether. I use the word business intentionally as outside of the religious context, the civil function of marriage is essentially a business partnership wherein two (or more in UT) people agree to cohabitate and share expenses and the raising of any children that might result. Let the state create civil domestic partnerships that perform the same function regardless of the relationships of those entering them. Might even allow for limited time contracts as well with provisions built in for the dissolution. Heinlein fans would recognize the kind of setup I am imagining here. The only real interest Gov't has in marriage is ensuring the care of minor children and the assignment of property & debts.
What does or does not constitute a "marriage" is the realm of the individual churches as long as all parties are consenting and have reached the age of majority.