settling instead for recognizing “unions” for both gays and straights? If we do that, then is “marriage” a status reserved for church members, thus excluding from the institution not only the fraction of the population that is gay, but also the growing percentage of married couples (like my wife and myself) who were not married in a church and/or are generally “unchurched”?

IMHO this is a question best left to society as a whole, and out of government.

That is, let's say government gets out of the “marriage” business altogether, and starts issuing licenses for “unions” (a contract that confers certain rights and responsibilities) between any two consenting adults.

It will then be up to society to decide which unions are marriages and which aren't. If Bob and Janet have a church ceremony in which they declare their undying love and devotion to each other, and sign a union certificate, it won't be hard to convince people that Bob and Janet are married.

If Peter and Sarah do the same thing, but in a public park with a JP instead of a church and minister, there'll be some people who'll deny that they're really married, but it'll still be an easy sell in most cases.

If Sam and Max have been living together for five years, and eventually formalize their relationship with a public ceremony, people will probably be evenly split over whether their legal union is a real marriage.

If Britney and Jason get drunk in Las Vegas and wake up the next morning with a signed union certificate, well, it's up to them and the rest of us to decide whether it should be called a marriage or not.

The boundaries will shift as society fluctuates between liberalism and conservatism. Though I think the overall trend would be toward liberalism.