Question about gay marriage in California: proposition 8?

I have been reading about proposition 8 in California. Apparently this would ban gay marriage and undo all the gay marriages that have taken place since California made it legal. I have read that groups are pouring millions of dollars into this campaign, on both sides of the issue. My question is, what is fueling the groups who are trying to negate these marriages? Just because the law says gays are not married they'll still live together, love each other and sleep together. So why deny gay people the tax incentives straight couples get? Don't gay people pay taxes too? How is this a question of legality? If the church doesn't want to recognize it, who cares? But how is it that a straight person should define the laws of a gay person?