President Obama's declaration in favor of gay marriage is right in principle. Americans who promise to live together as a monogamous couple should enjoy the same legal benefits, whether they are heterosexual or homosexual. The Constitution's provision of equal protection under the law enshrines this idea, and it should be extended to gay and lesbian people. However, we would achieve that result through civil unions for all couples, gay or straight, rather than through marriage.
Marriage traditionally has been a rite of the church, and it should remain so. The government should not have a hand in it. Rather, the state should sanction a contract between two people as a civil union. If, in addition to a civil union, a couple wishes to be married by a religious institution, that should be a matter between them and their community of faith.
Other nations have adopted this model and it functions quite well.