The short version of my opinion is that I support gay marriage, for both political and social reasons.
I talk, sometimes, about how my parents have no friends and no social life. This isn't strictly true, because my mom's in the church choir and has friends there, but my father really doesn't have any friends in the area. However, that wasn't always the case. When I was younger, they used to double-date with a couple they knew in town, one of whom was a co-worker of Dad's. They still tell a story about a memorable trip to a Chinese restaurant. The other couple helped out with our yard work, brought toys and books to me and my brother, and invited us all to dinner. When they moved out of town, my parents stopped going out in the evenings.
Now, obviously, they were a gay couple, or the story wouldn't be relevant. Wilbur and Bob have been together longer than my parents have been married. Last I heard, they and their dog were living in Provincetown, and Bob had become an interior decorator. How stereotypical can you get?
It's thinking about Wilbur and Bob that really pisses me off when I hear people saying that gay marriage is a threat to the sanctity of marriage, or calling the various bits of legislation that have come up Defense of Marriage. Leaving aside whether it is right or wrong, legally or morally, how exactly does this stable, faithful couple (or the others like them) threaten marriage and an institution? My parents' marriage was certainly never threatened by them. I don't think we would see a sudden rush of people leaving hetero marriages for gay marriages - most gays in hetero marriages end up leaving anyway, sooner or later. I can see the argument that no-fault divorce, or divorce at all, threatens marriage. Certainly a culture tolerant of infidelity threatens marriage. There are a lot of cultural factors that can threaten marriage, in fact. But I don't think that people who want to be married and make it work are among them.
As for the sanctity of it - well, let's go back and separate some elements here, shall we? A hetero couple that wants to marry has two basic options, the "church wedding"  or the civil ceremony. In essence, either they recognize the state government as the authority, or they recognize a higher power, which I shall refer to as God without intent to exclude other perceptions thereof. 
Gay marriage in the context of a church wedding really becomes a matter of the belief structure of that faith, except for where it is linked to the civil system. Some churches will never permit it, no matter what the state may say. Others have long had commitment ceremonies, blessings of unions, or honest-to-goodness weddings for gay couples, regardless of whether they are recognized by the state. I could be wrong, but it seems likely to me that the former are unlikely to have as many gay members as the latter anyway. In the long run, the religious validity is in the eyes of God. If we could know for certain what the right answers are, there wouldn't be multiple religions anyway. So, perhaps God doesn't approve, and gay marriage does violate the sanctity of marriage as an institution. If that were the case, A) it's been happening a long time, and B) the laws that get passed aren't going to change the positions of the churches or the position of God. And look, we're back to the First Amendment question from the last rant: "Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof..." Guess why this has been on a state level, folks. My own personal belief on the subject: if God had a problem with homosexuality, he wouldn't have created it. If God had a problem with gay marriage, there wouldn't be homosexuality in species that mate for life. It exists in nature, in animals who don't consider the possible morality of their acts. Perhaps I'm silly to extrapolate from animals to humans, but that's just how I see it.
Oh, yes, and then there's "tradition". They wish to claim that marriage, and always has been, between one man and one woman. Well, Islamic tradition is that a man may have as many as four wives, as long as he can support them all and treats them all equally. The Mormons have a history of polygamy, although they have repudiated it. Lots of traditions exist outside of the Judeo-Christian. Admittedly, the U.S. was founded as and continues to be a Christian nation. However, if we are claiming to be tolerant, and to have no state religion, then perhaps we should recognize that not all moral and ethical systems have the same premises. Also admittedly, the only examples I can produce without research are of one man, multiple wives. I don't have examples of the reverse, nor of the subject at hand. However, I'm willing to bet they exist.
So, if we take the religion out of the question, we're left with a secular, political question. If we do not fall back on religion and tradition, then to me, it appears that this is a simple case of gender discrimination. If you do not permit a man to marry another man, when they would be free to marry if one of them was a woman, you are discriminating on the basis of sex. If you extend health benefits to a long-term boyfriend of your secretary, but not to the girlfriend of the woman sitting next to her, you are discriminating on the basis of sex.
Some relatively progressive areas, in the hopes of warding off the whole issue, have established domestic partnerships as an alternative to marriage that is permitted to gay couples. On the one hand, I hate to denigrate it, because it is better than nothing everywhere, and in some places, it approaches what it should be. However, you would think that the civil rights movement of the 1950s and 1960s would have proven that "separate but equal" isn't. If it were truly equal, why would it have to be separate?  In the case of the domestic partnerships, from what I've read, they aren't truly equal, because they don't convey all the same benefits as a civil marriage. Until there is no reason for a hetero couple not to choose a domestic partnership instead of a non-church marriage, then the domestic partnerships aren't equal. And if it's really that important to make the word marriage only apply to a hetero couple, then make marriage the church ceremony, call the existing civil marriage a civil union and let everyone have access to it, and get rid of the partial solution altogether.
My friends and I used to play wedding after church on Sundays, just as people play house or school. We would go out to the church gardens, and have a procession. Jess was always the bride, and Melissa the groom, and I was the priest. Looking back on it, I wonder if it was a trifle prophetic. Melissa is unlikely to ever marry a man, but there's a decent chance she may marry a woman someday. I have often felt a strong inclination toward the priesthood , and while I don't have a true calling at present, I still believe that the answer I've been given is not "No" but "Not yet." Perhaps what I'm waiting for is a change in the laws, or in the accepted custom, that would make more of my beliefs commonly acceptable. I've often thought wistfully that I would like to marry my friends (as in perform the ceremony), although as time goes by, my friends marry around me, and I have not found a true vocation, that seems less and less likely. Maybe what I'm waiting for is the chance to perform the ceremony for my gay friends, and all the controversy going on at present is why I've been kept away from it.
Maybe it's nothing more than a strange set of ideas from a strange woman.
 One of my religion classes was a Ritual Studies class, focused on the ritual of marriage. It included traditions, symbolism, sacred texts, and romantic literature, from several world religions. I don't really remember how much information we actually got, but I wanted to take a look.
 I use "church wedding" in the most inclusive sense possible - a pagan ceremony in the garden or on the beach under the stars, just as much as the wedding in The Sound of Music
 Not only do I qualify everything, these days I do it in pseudo-legalese. *sigh* But seriously, I know my terminology doesn't apply to everyone, but I'm trying to get at root concepts - the similarities, rather than the differences. I think everyone knows where I'm coming from, but I also know that the one time I don't specify will be the one time that somebody takes offense.
 I don't think all orientation discrimination is a subset of gender discrimination, but a significant subset is - and it's the first and easiest foothold in the battle against it. The other major subset has to do with reactions, harassment, and a lot of other subjective elements, and is harder to isolate and to fight, except by general change of attitudes over time.
 The only persisting separate-but-equal institution that the overwhelming majority will support wholeheartedly? Public restrooms. Even then, most of the time they aren't equal. Women's rooms sometimes have larger capacity, and are sometimes a little nicer. They are more likely to have baby-changing tables. Men's rooms have urinals.  The restrooms that are equal are generally the ones that are equally unpleasant or inconvenient for everyone.
 Urinals for women do exist. I've seen them. Women won't use them, though. And they don't actually take up less space, nor is it faster to use them, so that's not strange.
 I'm Episcopalian, so yes, I mean priest, not minister or pastor.