Once again, rumors of an interfaith marriage between a Christian (a former one) and a Muslim have sparked riots in Egypt. Thirteen people were killed and two churches set ablaze in street fighting between Muslims and Christians – called Copts in Egypt – over the weekend. Hundreds were injured.

A human-rights expert I talked to said the rumors are a pretext for Muslim fundamentalists to incite sectarian strife. Interfaith marriage is not the root cause of religious violence, explained Dwight Bashir, at the US Commission on International Religious Freedom in Washington. Indeed, both the Coptic and Muslim religious organizations in Egypt forbid interfaith marriage.

Understood. But I can't stop thinking about this bloodshed and its connection to marriages of mixed faiths, especially given the trend toward interfaith unions in the United States.

According to the 2008 US Religious Landscape Survey by the Pew Forum on Religion & Public Life, 37 percent of American marriages mix faiths. "American Grace," a 2010 book by co-authors Robert Putnam (of "Bowling Alone" fame) and David Campbell puts the percentage at more than half. Whatever the exact proportion, the rate of interfaith unions in the US has grown rapidly in the last two decades, from about 25 percent.

Many reasons figure into this trend. The religious divide that came as a reaction to the free-love '60s has been replaced by a political divide. America is still a religious country, but it's getting more secular, with 16 percent declaring themselves "unaffiliated," according to Pew. And then there's just the basic makeup of America as a diverse and open society: People of different religious beliefs work together, sit side-by-side in school, live next door to each other. When they become friends, the religious divide fades.

The American trend doesn't make interfaith marriage any easier on a couple or any less complicated as a theological question. But couples that share similar motives and aspirations, similar values if not faiths, are attracted to each other nonetheless. Religions are having to adapt. Last year, Reform rabbis in America acknowledged intermarriage as a given that requires increased outreach and understanding. It was a switch from the previous view of it as a threat to Jewish identity to be resisted. The Pew survey found that 27 percent of Jews marry outside their faith; others say nearly half do.

Intermarriage is possible, of course, because wedlock in the US is regulated by civil law. That's not the case in Egypt or elsewhere in the Middle East where it is regulated by religious establishments that forbid marrying outside the faith (conversion is a different matter). Tiny Cyprus, the closest country to the region that allows interfaith civil unions, has become a wedding magnet for couples of different religions.

But human rights advocates in Egypt are pushing for "personal status" laws that would allow civil unions as well as divorce. As a democracy, Egypt should move in that direction, setting an example for its neighbors. How quickly its culture would catch up with a new law is another matter.