I think that voter attitudes may not be a critical factor soon.
Since the churches offering the sacrament of marriage to same-sex couples are not by any means cults or non-religious groups posing as churches to obtain legal or tax benefits, but real honest-to-God churches with a long history, this throws same-sex marriage into the category of a religious sacrament practiced by some churches and not practiced by others. For government to favor the churches that prohibit it over the ones that offer it, is clearly a violation of the anti-establishment clause of the First Amendment.
The simplest resolution is for the Supreme Court to decide that state-sanction of some church's religious rites fails the third "Lemon Test": It is excessive entanglement of church and state. If they seperate existing marriage into its civil and religious aspects, that cures all problems by untangling this Gordian Knot with one clean stroke of the ax.
States could regulate and record their civil unions, and give full faith and credit to other state's civil unions. Churches could perform or refuse to perform their religious rites as they see fit, with no interference from the government. (But of course, if you want the states or federal government to recognize your relationship, you do need to fill in the paperwork for a civil union just like our current use of "marriage licenses".)
The implementation is simple: Every existing legal marriage is already recorded in government records. The title of those records just changes to "Civil Unions". The religious validity of existing marriages could be affirmed automatically by some churches, while others might require actual membership, study, counseling, additional religious oaths or ceremonies, or whatever they like. (We call that "Freedom of Religion!) By the way, plenty of churches (eg. the Roman Catholic Church) already do this.
Future couples seeking civil union or civil union plus religious marriage would just follow a slightly different procedure in the very same government office where they now obtain a marriage license, in order to register their civil union.
Everybody gets what they want: People who want a civil union, get one. People who belong to a church that offers them and their partner a Marriage Ceremony, can get that if they satisfy their church's requirements. People who want to belong to a church that does not offer same-sex Marriage Ceremonies, can do that. Churches are free to determine their own religious tenants and rites, without government interference.
The only people who would be unhappy, would be those who desire to control the religious rites of churches other than their own. And they can bite me, to put it politely.
