Here's what I think.
Yes, allow it. I would dearly love to be able to marry my irl boyfriend in game, however, he picked a pretty catgirl, same as I did. I don't think either one of us plans on changing, and I imagine a LOT of other people are probably in the same situation.
Don't call it marrage, as that has super special meaning for Christians and such. Give it another name, like the ones suggested earlier in the thread. Say you have it enabled, weddings, happy party union times, the whole nine yards, but don't specify if it's one, the other, or both. If you don't like it, don't try to marry your best friend to see if it works, or you are stuck together for a good long time. ;P
Make a devorce quest, make it as expencive as can be. Make people think twice about having their unions. Make sure that they know what they are getting into, then we don't have people complaining about the above situation, and if they do go through with it then, they deserved it. :P
And hell, even if SE DOESN'T want to make it so GMs go over the cerimonies, at LEAST give us the option to craft the things needed. Then, people can have a cerimony where ever the crap they want to. Doesn't even have to be the specified happy union building.

I totally understand the big deal, it could backfire massivly, but it's a game for god's sake, get your panties out of a bunch and just play how YOU want to. If I wanna marry my boyfriend's character, that is no worry of yours, I'm not gonna show off or anything after the fact, it's just a nice, meaningless addition to a game we both love. :3