Just to address all of those who are saying "stop talking about this, it's a game, no one cares if you're allowed to marry!"...
While I agree that allowing or disallowing gay marriage will not directly in and of itself affect anything in "real life", I cannot say it will have no affect on the people who play the game— and not just gay people being offended because they can't seem to escape discrimination, even in an escapist fantasy.
Whether we like it or not, our entertainment can often influence our decisions and opinions of things we see in real life. As a gay rights activist, the problem has rarely been in making a logical case for gay marriage. We win pretty much every debate we get into, any outside observer could clearly see that.
No, the biggest problem is fighting the stigma that gays and gay marriage is weird and unfamiliar. Most people aren't against gay marriage because of any strong opinions, but because for decades gays have been portrayed as pedophiles, as diseased sexual vagrants, or as people who are just confused about who they are and need help from a higher power.
That stigmatization has a lasting effect on people, and very often the only way to combat it is letting them get used to gay people, and see for themselves that the stigma is wrong. Often it only takes having gay marriage legalized in your area for people to realize that gay marriage doesn't bring about a dystopian world where people run around in the streets buck naked and rape anyone who passes by.
If SE bans gay marriage in game, it could reinforce the idea that gay marriage is strange and foreign, not just to the people who play the game, but to anyone else who sees the developers of yet another MMO that won't allow gay marriage in their game.