Prop 8 groups are single issue donations. A donation means exactly and only that you agree with the cause of anti-gay marriage.
Politicians are many-thousand issue donations. I know of exactly zero people who I agree with on everything. A donation to a politician can have thousands of motives, and even be done in staunch disagreement about certain issues (especially in the case where their opponent is the same on the issue you disagree with and worse on everything else)
On net I'm conflicted about the whole thing. But I do think that people who value tolerance should be intolerant of the intolerant.