Like I've said before: A big mess.
The root of all that mess is the intertanglement of government and religion. Government sanction of a religious rite puts government directly in the middle of a conflict between the religious beliefs of some groups versus other groups, and that is expressly forbidden by the Constitution.
The only way to untangle this Gordian Knot is with one bold stroke: Let government administer civil unions, which would be required to have a relationship reconized by government agencies, tax code, law, etc. Since there would be no religious aspect to these civil unions, there should be little objection to offering them to all adults of sound mind regardless of their genders. That's what is required for Equal Treatment Under the Law.
Then let religious organizations offer any sort of Marriage Rites they like to their members or the general public. A church Marriage Ceremony would have no legal standing, but churches (et al) could still perform any sort of marriage rites their creed required. That's what is required for Freedom of Religion.
