People who don't think they should fight for something if it doesn't affect them directly. That's why Proposition 8 passed in California.
Shit, I didn't know it passed... Goddamn. This sort of shit really pisses me off.
At least the wording of the amendment doesn't specifically nullify all same-sex marriages made before the amendment.
I'm not sure how that works in the US, but Canada actually has a section of the constitution that says that nobody can be prosecuted for something that was legal when they did it, but is no longer legal. That would be pretty good protection for those marriages if that had gone through in Canada.