Cardinal Francis George of Chicago spoke Feb 23 at a gathering on the campus of Brigham Young University in Provo, Utah. He became the first Catholic cardinal to speak at the Church of Latter-day Saints university, speaking on "Catholics and Latter-day Saints: Partners in the Defense of Religious Freedom" as part of the Mormon school's forum series. The aim of the talk, of course, was to highlight the commonality of each religious institution's stance that marriage can only be between a man and a woman -- and to deny that gays and lesbians have a right to wed.
What struck me was what odd bedfellows Catholics and Mormons make around the issue of marriage. The Church of Jesus Christ of Latter-day Saints describe themselves as Christians, because they worship Christ, honor the Bible as one of the books that they use for faith formation, and they even have the word Jesus in their full name (For more info, check out this page  from the official lds.org Web site). Perhaps sociologically, Mormons don't appear much different than other Christians. However, theologically they are very different.
The Roman Catholic church does not consider Mormons to be Christians. The Congregation for the Doctrine of the Faith issued a statement  in 2001 that declared Mormon baptisms to be invalid Christian baptisms. The statement was written under the papacy of John Paul II and was signed by then-Cardinal Joseph Ratzinger, the current pope,.
Thinking back to the talk in Provo, there were many Mormons and Catholics in attendance, united around the drive to deny equal rights to gays and lesbians through marriage. It's plausible that a Mormon and a Catholic could have met at this talk, fallen in love, and gotten married themselves. What would this mean though?
There are irreconcilable differences, as noted in a document the Institute for Religious Research  published in 1999. Roman Catholics believe in monotheism; the Mormon church teaches that there are many Gods. This article points out that people can even become gods and goddesses themselves in the celestial kingdom and have children that worship them. Likewise, the Mormon church teaches that the Father, Son and Holy Spirit are three separate Gods. Of course, the Roman Catholic church teaches that these three distinct persons, the Trinity, are one God.
I have no idea how a Mormon and Catholic would raise children together or what they would teach their children about faith, theology, and marriage. In fact, if a Mormon and Catholic wanted to be married by a Catholic priest, they would follow the same ritual as a Catholic marrying a Rastafarian or an atheist -- the rite of a Catholic marrying a non-Christian. It seems fundamental that both traditions should agree on whether they are Christians before they decide who is allowed to marry and raise a family. Marriage is an intimate connection between two people, and issues like this are bound to come up.
I do want to express the caveat that I think it's phenomenal when diverse groups come together around social issues. I see strong connections among interfaith groups working on issues of economic justice. Jews, Muslims and Christians have strong theological understandings of the sin of poverty. These groups may not agree on everything, but at the very least, they can find commonalities in what they teach about the economic rights of each person.
The gathering in Provo is driven by a different agenda, one rooted in either fear or hatred of gay and lesbian people and their desire to marry. Usually, lessons from faith on ethical issues is meant to better the common good. I can't imagine the conversations between these two groups being too deep, because their theological differences would get sticky fairly quickly. The Mormon-Catholic partnership to "defend religious freedom" probably isn't meant to last. What happened in Provo was a shotgun wedding between conservative groups of Mormons and Catholics. Sooner or later, both groups will wake up from this shotgun wedding and realize that their long-term agendas about how to teach the faithful aren't meant to go together.