I just came across a great article by Jennifer Roback Morse on the the persecution which will result from legalizing homosexual marriage.
She points out that legally recognizing same sex marriage won't just affect the few homosexual couples who wanted to be married in the eyes of the state, but will lead to the persecution of those don't agreement with or support the practice.
She points out that recognition of homosexual marriage in Massachusetts forced Catholic Charities in Boston to close down their adoption program. A Methodist group in New Jersey lost part of it's tax-exempt status because it refused to sanction a civil union ceremony. A photography in New Mexico, who didn't want to take pictures of a lesbian marriage-type ceremony, was hauled before the state's human rights commission. And in Quebec, a Mennonite school was told they had to teach that homosexuality is merely another acceptable lifestyle.
I'm sure there are lots of other examples that can be found.
As I and others have said, the militant homosexual movement is the biggest threat to religious liberties in our nation. It's also an enormous threat to the foundational institution of our society -- marriage, because it actively seeks to define it out of existence. While divorce and cohabitation are enormous threats to the institution of marriage, there's no organized effort seeking to promote and encourage divorce and cohabitation. Homosexual marriage is a different story.