You should support same-sex marriage if you are a conservative.
Keeping government out of the private decisions of citizens is a fundamental tenet of conservative thought. If you are genuinely a conservative, you should be as concerned with civil liberties as you are with financial regulation. If you believe the government has no place in forcing you to pay taxes, because you don't want to be forced to support social programs, then it only follows that you should also understand that denying the right to marry to some citizens and not to others is unjust and not free. It represents and unwarranted intrusion into the private lives of citizens; just the sort of unwarranted intrusion that government has no place mandating.
You should support same-sex marriage if you are a Christian.
There is no Biblical marriage in America, and there never has been. "Biblical" marriage recognizes polygamy, requires that rapists marry their victims and never be divorced, forbids divorce all together, and permits husbands to sell their daughters into slavery and take captives from war and forcibly marry them. We have always recognized that marriage in America does not conform to Biblical strictures. And we have reformed marriage many times throughout American history, and always for the better. From no-fault divorces, to striking down miscegenation laws, America has steadily changed what marriage means throughout its history. Permitting same-sex marriage is no more objectionable than permitting mufti-racial marriages.
You should support same-sex marriage if you are a heterosexual.
Heterosexual marriages will not be diluted, threatened or diminished by same-sex marriage. Indeed, it's just the opposite. By recognizing what an important part forming new families has in our society, you strengthen the institution. By allowing more people who want to be married to marry, you permit more lasting social bonds. The fact is, if you're heterosexual, gay people don't want to marry YOU, they want to marry each other. By recognizing their right to do so, you create an America in which more people are in long-lasting, stable relationships.
You should support same-sex marriage if you are a liberal.
Social justice is an important tenet of liberal politics. Liberals lead the charge in the Civil Rights movement, and have continued to do so in combating this prejudice. I suspect I'm preaching to the choir here, but in case I'm not - consider that it is as unjust to discriminate against same-sex couples in the field of marriage as it is to discriminate against gay people in the workplace.
Marriage is not only about procreation.
Procreation is about procreation. More people than ever are having children outside the bond of marriage. Some of them are even gay people, having or adopting children! By reducing the number of unmarried people, you reduce the number of children born or raised out of wedlock.
In addition, people who are heterosexual but have no intention of having children get married all the time. Do we forbid the elderly, or the barren from marrying? We do not - and that's because marriage is not solely about having or raising children. When two people want to permanently join their families, fates and fortunes, they get married. Whether or not they have children, a new family - a stable family, ideally - is formed. More stable families is good for America. You should want that.
Marriage is an important and basic human right.
Every culture in history has recognized that people will marry, and have provided special privileges to married people that unmarried people do not enjoy. Something that is important to every human culture in history is clearly a basic human right.
There is no compelling reason to forbid same-sex marriages
I have yet to hear any explanation of forbidding same-sex marriage that doesn't boil down to bigotry. Homosexuality is not corrupting the morals of America. Even if your faith tells you that homosexuality is a sin, there are plenty of other sins that are doing demonstrable harm to the entire world. Greed and selfishness have savaged the world's economy. Hypocrisy is not only rampant in modern politics, it's simply the order of the day - no one expects that anything should be different, much less holding hypocrites accountable for their deeds. Far more heterosexual people have spread and transmitted the AIDS epidemic globally than have gay people. Why is this "sin" the only one you single out for special punishment?
Religion is not sufficient reason to oppose same-sex marriage.
If you can not conclusively prove that homosexuality is a moral failure without referring to received instructions from your faith, you can not legislate against it in a a society composed of people of many faiths or no faith at all. Note that there are far more prominent sins in the Christian hierarchy that are not illegal. No one has yet been jailed for disrespecting their mother and father, nor has anyone been imprisoned for idolatry or taking the lord's name in vain. Why, then, the singular focus on the so-called "sin" of homosexuality? When work on Sundays, idolatry, covetousness, lying, polytheism or any and all of the other tenets of the Decalogue have been made illegal and abolished; when lack of charity, lust for wealth, divorce, and all the other strictures on people's behaviors of choice have been abolished - then, and only then, should you consider outlawing homosexuality and denying the right to marry to same-sex couples.
But the fact is that even at its inception, America was a nation of many faiths, and will always remain so. The government is a secular institution, and religious prejudice has no place in informing or determining public policy.
No matter who you are, permitting same-sex marriage will not harm you, or anyone you love or care about. But denying same-sex couples the right to marry is, right now, harming someone you love and care about, no matter who you are.
You should set aside your prejudices, and speak up for fairness, for compassion, and in defense of simple human dignity. If you already support same-sex marriage, don't be afraid to speak up. Your voice matters.