I'm all for monogamy (tried it the other way; doesn't really work for me). It's in more the marriage part that I'm lost. I get the IRL argument for it (there are legal benefits and all) but does the ceremony itself really matter to people in characterising their relationship? Again, genuinely asking.
I think the official sanctioned marriage concept makes it sort of clear and binding that the people are committed to monogamy and the sort of each one for the other only kind of thing. In certain states it used to be a crime to commit adultery for example, which meant you had to be married. Life partners (which ok, be life partners if you want) doesn't really have any associations one way or another. You could be a life partner and still have people on the side here and there, etc, there's more of a feeling that you can get out of it.
Nowadays, well maybe it doesn't have quite the same connotations or impact, of course. But, still though, I think ceremony and such offers people who want to demonstrate to the world and themselves that they are invested in being monogamous and want to make that commitment a chance to do that. It's an opportunity for both people to show the other party that they mean what they say. Maybe (at least a little bit) the feeling that they can't get out of it is what they're seeking?
If there's some other world where some other form of connection (Life partner, whatever) carries the same weight and connotation it could be a feasible alternative to making that demonstration.
And plus hey the excuse to dress up and all the decorations are cool also. I went to an in-game wedding in FFXIV actually and it was pretty amazing, for all kinds of reasons, not nearly as dramatic or interesting if two people just one day go "hey, we're partners right?" and then someone else goes "yeah."
We can't use the term representation when talking about marriage, Kefka.
It is "disrespectful to women and the LGBT community asking for representation" according to Allan Schumacher. 
What? Why?