Republican politicians have three concerns about gay marriage besides safeguarding the institution of marriage. One is that the religious right, a powerful constituency, is dead against it.

But, if the high court decides to strike down laws against same-sex marriage — including California’s Proposition 8 and the federal Defense of Marriage Act — no church will be forced to perform such marriages. This will be a civil matter that religious institutions will be free to bless or not, as they choose.

And, theologically, as Sen. Rob Portman, R-Ohio, put it so well, “ultimately, for me, it came down to the Bible’s overarching themes of love and compassion and my belief that we are all children of God.”

The fact is, Jesus had nothing whatever to say about the subject. Biblical condemnations of homosexuality are contained in the Old Testament, especially Leviticus, and in various letters of St. Paul, who listed it with numerous other sins — adultery, fornication, etc. — that conservatives have long since given up trying to control by law.

To be biblically faithful, they’d have to advocate putting gays to death (Leviticus 20:13). Then, again, they’d be obligated to offer them Christian forgiveness, too. (Hebrews 12:7).

The next conservative objection is that a decision legalizing gay marriage would be an act of judicial overreach and a breach of the principle of federalism.

This isn’t a trivial case — though I can’t see how the court, on equal-protection grounds, could possibly sustain Article 3 of DOMA, which denies federal benefits to legally married gay partners that are available to heterosexuals.

Personally, I think marriage equality is a constitutional right. But I could see the court refusing to strike down the results of California’s 2008 popular referendum — especially since it passed then by just 52 percent and would undoubtedly be reversed in a new referendum today.

I could also see — though not agree with — a conservative majority sustaining DOMA’s grant of authority to the states not to recognize other states’ legalization of gay marriage.

That would perpetuate a crazy-quilt system across the country — only nine states plus D.C. recognize gay marriage; the rest don’t — but the clear drift of public opinion is toward tolerance. So, over time, only fewer and fewer states would be left not permitting gays to marry.

Which brings us to the major reason Republicans don’t want to do the right thing — the fact that 69 percent of Republican voters oppose gay marriage. GOP politicians either are representing the views of their base — or are afraid of its retribution.

But, Republican leaders really ought to be afraid of other numbers. According to a Quinnipiac University poll, all groups destined to dominate the political future are pro-gay marriage: Hispanics, 63 percent to 32 percent; young voters, 62 percent to 30 percent; college-educated whites, 59 percent to 32 percent; and white women, 50 percent to 40 percent. The GOP will have to take action on immigration to erase its suicidal standing with Hispanics, but perhaps its leaders should hope that the Supreme Court gets it out of trouble on gay marriage.