Because no one's rights should be subject to a voter initiative. (Bumper sticker idea: Who voted on YOUR marriage?)
Go here for the sequel .
Scarcely 2 months till the election, folks! This is no time to be getting complacent. There is no way of telling whether the polls are accurate (people may tell the pollsters they are opposed to Prop8 because they are ashamed to admit they support bigotry; this is called the Bradley effect and is a concern for national polls as well). You don't have to be a Californian to contribute to the cause. After all, the Knights of Columbus, Focus on the Family and the Mormons are pouring money into the pro-8 campaign.
The Mormons and the Catholics have mobilized people to walk the neighborhoods and promulgate lies (such as saying that all churches will be forced to marry gay folks). Howard Ahmanson himself has put in $400K. You remember Howard, right? Of course you do--he's bankrolling the schism in your church.
Please keep IT's marriage legal! Oppose Prop8.