Because Christians, like everyone else, like to be distracted from accepting responsibility for their own problems. It's an unfortunate part of the human condition, and Christians are humans.
Divorce in the particular Christian "denomination" that I go to is not as high as the "secular" world, but the margin is only a few percent. I believe, if my memory serves me right, you could probably argue that the difference falls with the margin of error. I would wager that Christian churches as a whole have divorce rates that approximate the secular divorce rate.
But, it's so much easier to say "Gay marriage will destroy society!" than it is to say, "Why are our marriages failing?" It's easier to say "Hollywood is destroying the family" than it is to throw out your television or quit going to the movies. It's so much easier to write your congressman about Adam and Steve's desire to be married than it is to sit down with your wife, knee to knee, and commit to do whatever it takes to make your marriage work.
I feel fortunate that I attend a church that focuses on honoring God and strengthening one another--not kowtowing to the latest political/moral movement. I've been a member there for four years. Never has prayer in public schools been preached about. There have been plenty of sermons about your responsibility to educate your own children in the faith. Never has gay marriage been touched on. There have been plenty of sermons about strengthening your marriage. Never has their been an endorsement for or against a candidate for public office, although the preacher has reminded us to get out and vote. The gay marriage ban wasn't even discussed when it came up for a vote, last year.
As a Christian, I do believe that marriage is a sacred institution set forth by God. And, I believe that marriage is intended for one man and one woman. As such, I can't support the idea of "gay marriage." I see marriage as a socio-religious contract. Government sees marriage primarily as a economic and legal contract. The solution that I feel is best for our society is a "divorce" between marriage and state governments. Let governments have civil unions that any two people can get into for economic/legal reasons. Let people, then, if they want to get married, go to whoever they see fit to marry them. Priest, judge, her best friend, whatever . . .