Benedicte Desrus / Sipa USA Ugandan gays and supporters attend a conference to promote homosexuals' rights in Kampala, Uganda, on Feb. 14, 2010.

The U.S. Supreme Court is mulling this week whether to walk down the constitutional aisle of gay marriage. The timing feels especially apt for American Christians, who this week are marking the start of the reflective, pre-Christmas season of Advent. Few groups have played such a key if controversial role in the gay-marriage dispute, and few have been as divided by it. (Most U.S. Roman Catholics support gay marriage, even though their bishops are among its most vocal opponents.) Now — with a re-elected President backing gay marriage, three states legalizing it last month and another rejecting a bid to ban it — Christians seem to be at a crossroads about homosexuality, much as they were a half century ago about civil rights.

So it might be helpful if Christians looked not at Washington, D.C., but at Kampala, Uganda, where an abominably homophobic “Christmas gift” is about to become law. The anti-homosexuality bill speeding through Uganda’s parliament right now — which that body’s Speaker has pledged will pass by year’s end as a “Christmas gift” to its backers — would impose draconian new punishments. Among them: a seven-year prison sentence for consenting adults who have gay sex, life sentences for people in same-sex marriages and even jail for those who don’t report gays and lesbians in their midst. Fortunately, Ugandan lawmakers say they’ve dropped the bill’s death penalty in cases of “aggravated homosexuality,” in which HIV is spread or gay adults have sex with minors. That article had prompted opponents to call the original legislation the Kill the Gays bill.

(MORE: Rick Warren Denounces Uganda’s Anti-Gay Bill)

Christians worldwide, including Catholics like me, should find the Uganda bill as relevant as it is repulsive. That’s because Christians in that East African nation — whose lobbying is generously funded by conservative U.S. Christian organizations representing every denomination from evangelicals to Catholics to Mormons — are perhaps its most influential supporters. In fact, the Uganda Joint Christian Council pushed parliament this year to expedite the legislation and “remain steadfast in opposing the phenomena of homosexuality.” One council member, Catholic Archbishop Cyprian Lwanga, opposed a similar bill three years ago, when the Vatican denounced “all grave violations of human rights against homosexual persons.” But since then, Lwanga seems to have decided that prison time for gay sex isn’t so grave after all.

Conservative Christians in more-developed countries like the U.S. will argue that they’d never advocate something as severe as Uganda’s bill. And some, like leading evangelical pastor Rick Warren, have, to their credit, decried that legislation. But I would remind them that 13 U.S. states (including mine, Florida) had antisodomy laws in their criminal codes as recently as 2003, when the Supreme Court finally struck them down. Many will insist that their opposition to gay marriage has nothing to do with how they feel about homosexuals personally and everything to do with their belief that it undermines the functional formation of, if not divine plan for, the vital family institution. I would point out that gay and lesbian couples have been raising perfectly functional, if not divinely acceptable, families for decades now.

No, the real question that conservative Christians from Florida to France to Fiji need to ask themselves at this point is this: By crusading to deny gays and lesbians the right to legally marry — by insisting that God doesn’t consider loving gay unions morally worthy of matrimony and therefore the state shouldn’t either — do they risk demonizing “the phenomena of homosexuality” as inhumanely as the Ugandans are? It’s of course a good thing that the Vatican has condemned the “abuse of homosexual persons.” But as a Catholic, I’m all too aware that Pope Benedict XVI has also said that saving humanity from homosexuals is as crucial as saving rain forests from lumberjacks. And that a Vatican spokesman, after last month’s pro-gay-marriage votes in the U.S., made the equally cruel remark that gay marriage is a slippery slope to polygamy. Don’t blame Ugandan Catholics if they’re getting dangerously mixed signals from Rome.

Still, conservative Christians will claim that St. Paul’s denunciation of homosexuality leaves them no scriptural wiggle room. But St. Paul also condoned slavery, and I think we can safely say Christianity has managed to wiggle out of that one, just as Jews today feel O.K. about ignoring the Torah’s edict to stone nonvirgin brides to death. Like everything else in life, religion has to evolve. If it doesn’t — if it remains as rigidly static as so much Christian doctrine has so far in the 21st century — it risks the irrelevance it increasingly faces in the U.S. The Pew Forum on Religion & Public Life recently found that a record one-fifth of Americans, including a third of adults under age 30, aren’t affiliated with any religion today.

(MORE: Urban Ebola? Why the Latest Outbreak in Uganda Raises Worries)

If the U.S. Supreme Court takes up gay marriage during this session, many legal scholars believe the Justices will likely rule that banning it is unconstitutional. That’s partly because, they say, the high court will not want to appear as out of touch with history a decade from now as it would today if it hadn’t kiboshed the sodomy laws a decade ago. Eventually, hardline Christendom doesn’t just stand to find itself on the wrong side of history on homosexuality. It could also end up on the wrong side of Christianity, as Ugandan Christendom is this Christmas.

MORE: Is Gay Marriage Too Progressive for the French?