Marriage was already redefined when the Protestant "reformers" said it wasn't a Sacrament.
Then, Catholics and Protestants ignored Jesus' words that marriage was for life.
Then, Catholics and Protestants ignored Jesus' words that marriage after divorce was adultery.
Then, Catholics and Protestants ignored Jesus' words that sex outside of marriage was a sin (fornication).
Then, Catholics and Protestants ignored Jesus' words that sex should be life-giving.
Then, Catholics and Protestants ignored Jesus' words that sex is between a man and a woman.
When society doesn't see marriage as a Sacrament, when they don't see it as lifelong or life-giving, when they think they can just divorce and remarry, when they think they don't need to be married to have sex or raise a family - and even Christians approve and participate in these bad decisions - then of course it looks like we're being jerks by saying homosexuals can't get married. Through those eyes, marriage DOES seem like just a legal contract between two (or more) people in love and that protesters are just being unkind and discriminatory.
But that doesn't mean they are right and God is wrong. It does mean that we have failed to protect this Sacrament. Would we accept the Supreme Court changing the Sacrament of baptism, that it can be made with soda instead of water? Or that instead of using oil for the Anointing of the Sick, we can use Cheese-Whiz? Of course not. But because of our divisions, because of our unfaithfulness to Christ's explicit words, and because we gave to Caesar what was God's by allowing the State to interfere with marriage, we got the Supreme Court ruling that we deserved, the one that we earned.
If we don't like it, then it's time to be faithful for once and stand firm, in our personal lives, in our public lives, in how we raise our families, and in who we vote for. You know, try to be the 'salt of the earth' as Jesus said - otherwise, like Jesus promised, the only thing we'll be good for is to be thrown out to be tread upon.