I find it interesting that, when people invoke "natural law", they often do so in support of their own personal or religious beliefs and the status quo.
Arguing, for example, that the movement for same sex civil marriage wrongly compares itself with the movement for racial equality because same sex marriage, "unlike the civil rights movement, can be challenged on classic natural law principles" is specious and self serving; in fact, slavery, Jim Crow and anti-miscegenation laws were defended by the claim that "natural law principles" dictated the superiority of white skin over black, that an African American did not count as a whole person and surely, "natural law" would never allow a man of color to marry a white woman.
"Natural Law" was used, as well, to defend the subjugation of women; women's "natural inferiority" was invoked to justify denying women the vote, denying them property rights and allowing men to beat their wives.
It is a fact that the Supreme Court has called civil marriage a basic civil right, most notably in Loving v. Virginia (1967), but also in Zablocki v. Redhail (1978) and in Turner v. Safley (1987). Given that marriage is a basic civil right and given the Fourteenth Amendment guarantee of equal protection under the law, the state needs to show a compelling reason to justify denying marriage rights to a group of people. This is not to say that people who object to same sex marriages on religious grounds should be "banished from the public square"; if they have a non-religious objection to same sex marriage, let them make it. But, when the only objections to same sex marriage are based on a particular religious sect's interpretation of the Bible, those objections must have no bearing on civil law lest other religious sects with other interpretations be barred from the free exercise of their beliefs.
Bear in mind that civil marriage rights alone are in question; just as churches may now refuse to officiate at or recognize interfaith marriages, the rights of churches to deny religious marriages to same sex couples will not be violated, thanks to the establishment clause of the First Amendment. Also bear in mind that the definition of civil marriage, a contract between two consenting adults who are not already close relatives (marriage creates a close family relationship, so there is no need for people who are already close relatives to marry), will not change; allowing same sex couples to enter into the civil contract known as civil marriage will not in any way change the nature of marriage for heterosexual couples, nor will it open up marriage to children and pets who, of course, are not consenting adults.
Is there a compelling state reason for preventing same sex couples from marrying? People claim it will harm children, but I have yet to see a convincing argument for that. Numerous well conducted studies show that children benefit from growing up in a stable family environment regardless of the gender of their parents; this is why same sex marriage is supported by the American Academy of Pediatrics, the American Medical Association, the American Psychiatric Association, the American Psychological Association, the National Association of Social Workers and many other professional organizations. The fact is, same sex couples will have children whether they are allowed to marry or not; why, then, should their children be denied the legal protections currently enjoyed only by the children of heterosexual married couples?
Absent a compelling reason for the state to deny marriage rights to same sex couples, are there reasons for allowing same sex marriage? Absolutely; stable families provide both economic and social benefits to the state. Leaving aside the potential economic windfall as same sex couples book wedding venues, hire caterers, musicians and florists, people who are married require fewer governmental services when they fall ill or are otherwise in need because they have a partner to care for them. According to the CBO, while Social Security spending would increase as a result of same sex marriage, the government would save more on SSI, Medicaid and Medicare. Moreover, businesses are more willing to locate to states with large pools of talented workers, including gays and lesbians, which is why seventy companies, law firms and trade organizations filed an amicus curiae brief in Gill v. Office of Personnel Management, calling the Defense of Marriage Act unconstitutional. Companies that support same sex marriage include Best Buy, Costco, Delta Airlines, Ford, General Mills, Hilton Worldwide, Home Depot, IBM, Walgreens, Target, State Farm Insurance, Nordstrom, Wells Fargo and many others.
Legalizing same sex marriage will, of course, have no immediate impact on how people feel about homosexuality; I have no doubt there are still people who bristle at the thought of interracial marriage, after all. No law can change how people feel, but it can change behavior; religious conservatives may not like knowing that same sex couples would be able to marry, but they would no longer be able to prevent those marriages from taking place either in front of a Justice of the Peace or in a church or synagogue that supports same sex marriage. And it is, of course, a fact that several religious groups support same sex marriage, including Conservative, Reform and Reconstructionist Judaism, Liberal Quakers, Episcopalians, some Lutheran synods and Unitarian Universalists. By what right, then, should the state compel these religious groups to follow Catholic and Conservative Christian principles regarding marriage? Yes, there are religious arguments both for and against legalizing same sex marriage, and for that reason the debate must be kept in the realm of civil law; otherwise, we risk making Catholicism and Conservative Christianity the official religions of Minnesota.