Marriage is about as sacred as life. Which is to say, no one treats it as sacred, but everyone is willing to say it is, if that claim is being used to take away the rights of someone else.
First of all, as a legal contract, marriage has no PLACE being sacred. Churches can say and do whatever they want, but the legal institution of marriage is not sacred in any way shape or form, in fact, it's rather secular.
In addition I'm sure I don't need to go through the motions of pointing out the flaws of people who say "we can't change the definition of marriage", do I? That's been said and done before, but it just goes to show that marriage is a very malleable thing. It's shape and form have changed greatly over the centuries, and between culture and culture. As long as marriage no longer involves a man trading some goats and chicken for the ownership of a man's daughter, or two noblemen arranging the marriage of their children to promote a political alliance, I think it's safe to say that marriage is NOT sacred enough as to be beyond human alteration.