What's Threatening about Gay Marriage?

Thinking about this lately. It's clear from both NT and OT passages that God's ideal is permanent, heterosexual union (Mark 10, where Jesus refers to Genesis and makes his own pronouncement). But that's obviously not the trend in civil, secular society, regardless of widespread feeling that America is still a "Christian nation."

I have a problem with the "Defense of Marriage" camp. The idea that gay marriage is threatening to the institution of Biblical marriage seems a little silly. At the very least it's like closing the barn door after the animals have escaped. The greater threats to marriage have always been things like adultery, divorce, abuse, pornography, and such things. You know, issues and problems that can actually break up hetero marriage more than a same-sex couple tying the knot.