The first is that Aristophanes actually talks about homosexuals in the same terms as heterosexuals. If two men were originally fused, their separated halves will seek out lifelong romantic relationships with other men. If two women were separated, they will seek out enduring romantic relationships with other women. It is an important point because it is the only piece of Greek writing that explicitly explains the existence of what we call lesbians; Sappho actually doesn’t, although of course she’s important for lesbians as well.
It’s also unique in talking about homosexual romantic relationships. As everyone knows, male sexual relationships were common in Greece (and certainly in Plato), but they’re often more akin to older mentor/younger student sexual relations. The young man learns from the older man, who in turn has sex with him until he is old enough to grow a beard. What’s strange is that there is opprobrium in many Greek stories aimed at catamites, but many men like Socrates highly valued chaste love between male friends (thus called platonic love), and of course homosexual relations were common among soldiers because it was thought to strengthen their fighting bond.
As far as gay marriage goes, the story is important because it makes clear that there were men and women living together in enduring homosexual romantic relationships as far back as 385 BCE. It must also be noted that the oldest epic in Western history- the Iliad- has at its center a love relationship between Achilles and Patroclus. I suppose it’s pushing things a bit to call Achilles gay, but there is a clear difference between how he sees the slave girl, as property, and his buddy Patroclus, whose death breaks his heart.
The Symposium is also interesting in how it treats heterosexual romantic relationships. Aristophanes says that heterosexual men and women who find their lost romantic partner will then become adulterers and adulteresses. This might seem odd, but we have to remember that the Greeks did not marry for romantic reasons, but were in what we would call arranged marriages.
In fact, we should remember that the vast majority of married people in Western history have been in arranged marriages. This perhaps explains how many Western stories there are about adultery (or maybe not), but it is definitely still a theme in Shakespeare, roughly 1,9000 years later. The norm was always for young women to be married off by their parents. In fact, you still see this in the late 1800s, although romantic courtship is starting to win out, and arranged marriages start being portrayed as bourgeois calculation, as in Effie Briest. Romantic love and chivalrous love were often unfulfilled loves, and of course, the husband whose wife “put the horns on him” was a common comedic archetype, also popular with Shakespeare of course, and still is in many peasant cultures.
The point for gay marriage advocates is that the “definition of marriage” has already been changed in the West with the introduction of romantic marriage. There is simply no change that could be comparable to the idea that marriage should be the result of romantic love. Marriage used to mean the alliances formed by families for the betterment of their children. Now it means the bond of two people who are in love with one another. While Judeo-Christians are free to believe that gay sex is sin, it is simply irrational to believe that romantic relationships do not exist between homosexuals, or that a secular government should pass religious judgments on them.
My sense is that many homophobes see gay relationships as being solely sexual and not romantic; and thus different than heterosexual relationships. Therefore, they see no logical reason that gays would want to wed, aside from hostility towards heterosexuals. Of course, this is provincialism and wrong, as familiarity with the Western canon easily demonstrates. Whether they were bunkmates, buddies, old maids, lovers, sinners, or catamites, they have always existed and always will exist. Plato figured this out about 2,400 years ago.
Note: Greg's right- this was a glaring omission...