Marriage is an institution.
So was slavery, if I recall.
The terror tactics are being hauled out of the closet again. Gay marriage will be force-taught in schools, marriage is a sanctified institution between a man and a woman, gays already have the same rights, etc. Nothing causes a sudden deer-in-the-headlights tensing-up like the prospect of homosexuals reserving a few hours at the local chapel. Beware, America. We're going to have to invite them to our football parties, and they're going to rifle through our sock drawers and touch things, and our sons will commence a mysterious addiction to musicals.
The Sanctity of Marriage. There is no such beast, not historically. If you try argument-by-tradition, you'll fail; was not marriage typically for financial and social gain? If not, whence comes the concept of "dowry?" Whence comes the idea of "marrying up?" We as a western culture are still classist as fuck, even more so than racist and almost as much as sexist. This is a fairly modern concept, this "love" thing.
Continuing on this: sanctity of marriage? Do straight people do a bang-up job preserving this? With so many "my heinous wedding" shows on television, wedding horror stories, and a multi-billion dollar industry based on making your perfect day the object of envy? Are the couples on "My Big Redneck Wedding" reflecting the somber-yet-joyful institution of marriage?
Perhaps they are. Why not? They're marrying for love, after all... and so do homosexuals. You want loyalty? Participation in society? Contribution to the country's economy? Dedicated parenting? You've got it.
What else... Biblical support. People tried to use the Bible to support the anti-miscenegation laws that would have prevented me from marrying my wife. Interracial marriage wasn't struck down until 1967, the year we were born. Slavery also has biblical support.
"But I have no problem with interracial marriage." You used to.
Let's also look at at this way: gay marriage means that two people are getting together, presumably for life, and are less likely to continue in a "promiscuous lifestyle," if you accuse homosexuals en masse of such a thing.
We're urged to "protect marriage." What are you afraid of? What will change? A lot of people will be a lot happier, for one thing; equal insurance rights, spousal visits in the hospital, funeral arrangements without the extra barriers. Tap into your greed if you have to and think of the business opportunities. That multi-billion wedding industry will be more tickled than ever; gays will spend as much money on that perfect day as you will. Capitalism is gender-blind.
What else can be argued? There's going to be a slippery slope toward incest, bestiality and child molestation? Marriage is for procreation? Children cannot be raised by gay parents? Gay marriage promotes gay recruitment? Bring those on.
No on Proposition 8, please.