Any Actual Arguments?

Does anyone have an actual argument for why gays can't get married. Most of the arguments I've heard are more like underpants gnome arguments.

1. Gays get married
2. ?
3. Civilization collapses or something

Most of the argument centers around damaging the "institution of marriage" which before the debate came across my radar I didn't know existed. I knew marriage existed, otherwise I wouldn't be married, but I didn't know that there was an institution called marriage. Does it have a building? Hell does it even have a particular set of rules that apply to everyone. I keep going back to the old saying that you can't ever tell what its like inside of someone else's marriage. Some rules that seem inviolate for me are not for others. Marriages continue to exist after infidelity, madness, hell some marriages probably continue to exist even though both people in them are gay and of the opposite sex.

Marriage seems to be something between two people who decide to make their own rules. Even if you look at the traditional church vows there is a lot of wiggle room inside all those vows as to how they will actually be kept.

So where is this institution?

The other argument is that we'll be changing the traditional definition of marriage. I don't particularly understand why we should be so concerned about a words definition. Many words have changed definition. Awful used to mean full of awe, now means really bad. I mean if someone got up and screamed that pizza was and inviolate word and needed to be chained to one thing for all time most people who just stare quizzically at them wondering if there was anything else nuts that the person was going to say.

But the counter argument might be that pizza is not an important word. Marriage is an important word. Are we then arguing that society is based on words and language and nothing more stable? That its merely a collective agreement amongst individuals? If that's the case then there is no good reason that we can't change the shared consensus in particular ways. Its all subjective anyway if its just a shared consensus.

But marriage, my imaginary arguer argues, was instituted by God, it's in genesis. (I guess this is where the institution of marriage nonsense comes from but it seems that if an institution in this case is just something instituted the the arguer is really piggybacking a lot on the rhetorical power of the word institution which in my mind at least has a lot of buildings associated with it) Of course the normal counter argument to that is that the bible shows that someone can be married to many people even though there is only one favorite at a time. Serial polygamy. The counter argument to that I've heard is that most of the examples of polygamy, such as Abraham taking another wife and then having to send her away at the jealousy of his first wife shows that polygamy is bad. But here we get back to the problem of the utilitarian effects of different forms of marriage which just goes back to my original question, what awful effects would gay marriage have?