Republican ex-Congressman Tom Campbell explains in this article why California Republicans should vote NO ON 8. He does so much more eloquently than I could, so you should go right there.
What's really interesting is that the arguments for Prop 8 are exactly the same ones that were used to maintain segregation - the old "separate but equal". So, if you're against segregation, you should vote No on Prop 8. Notice how people are getting shipped into the state from across the country by national organizers - remember, this is a state proposition - so what happened to States' rights all of a sudden?
NO ON PROP 8!!!