Marriage equality is a touchy subject, in no small part because so many irrelevancies crop up in the discussion and 'muddy the waters'. I say, let's strip away the irrelevancies in order to discuss the subject plainly.
Let's eliminate one irrelevancy right away: Just because the Bible prohibits something, doesn't mean that civil government has any cause to prohibit it. The United States is a constitutional republic, not a theocracy. End of story.
That argument being unequivocal, the next absurdity which emerges from that camp is the idea that legalizing same-sex marriage must also legalize polygamy, since unfettered marital choice must allow the option of marrying multiple partners. This argument fails for one very simple reason: Gay couples are asking to be married under the same conditions as straight couples. One of those conditions is that a person cannot have more than one licensed marriage at a time. Polygamy is a red herring.
Then there's the "biological" argument: Since homosexual couples can't procreate, their marriage is invalid. Please go find a heterosexual couple without children and inform them of this, right away.
Then comes the 'conservative' legal argument: SCOTUS considering this case is akin to the Supreme Court considering Roe v. Wade, in that advocates are asking the federal courts to overturn an action of a state democratic process. I respond with the following:
2) Roe was an attempt- and a miserable one at that- to resolve an irresolvable conflict between two diametrically opposed viewpoints of personal liberty. The last time such a conflict of liberties occurred, half a million Americans killed each other. I won't address either the Civil War or the abortion debate; I mention them to point out that same-sex marriage is not an irresolvable conflict of this type. Person A's exercise of religious rights is not affected in any way by Persons B and C's gay marriage, except in the one circumstance I will address below. This is an issue of equal protection, plain and simple. A minority wants to enjoy the same 'privileges and immunities' enjoyed by the majority;
The one circumstance I referenced above, with respect to the exercise of religious liberties, is the prospect that individual gay couples or gay advocacy groups will use legal actions, or other means, to harass churches and businesses for choosing to not participate in same-sex weddings. This is not only a legitimate concern: it has already happened multiple times and it will continue to happen. For this reason, were I a legislator, I would refuse to vote for any gay marriage bill which doesn't include specific protections for those who choose not to participate. If you, the reader, support gay marriage but aren't of the same mind, then you are a hypocrite. We must preserve the rights of all individuals, including the freedom of individual conscience and the freedom of choice.
And then there is the "compromise" position: Recognize same-sex civil unions in all states. This sounds feasible and moderate enough, with one glaring problem: A civil union isn't a marriage! There are inequalities between marriage and civil unions- inheritance issues, immigration issues, health insurance and tax filing problems, etc. Even if these issues were all resolved perfectly, however, a civil union would still be less than a marriage. SCOTUS determined a long time ago that "separate-but-equal" was not a legitimate form of equality.