This would be a big old rant waiting to happen, but as luck would have it, Kip Manley's already gone and done it for me:
Defend marriage? You pathetic, deluded fools. Same-sex couples have been getting married all around you for decades, and they’ll keep on doing it, long after you’ve passed your little amendment. Men will kiss their husbands as you clap yourselves on the back, and wives will continue to feed each other cake, whether you will it or no. They’ve always had the love and the cherish and the honor, and the recognition of their friends and family, and nothing you can do will take that from them. Nothing. All you’ll manage to do is rewrite the tax code. Make it more of a grinding hassle to deal with insurance and wills. Keep loving families apart at times of illness and accident and death. Condemn children to needless, nightmarish legal quagmires. You will tarnish all our rings, and when we open our mouths to take our vows, we will taste ashes. —In order to save marriage, you will destroy it. Fools.
Yeah. What he said.