Our first amendment rights in this county give us the freedom of speech, but they also give us the freedom of religion. And, by extension, the freedom from religion. Eich's personal views on gay marriage shouldn't trump the rights of anyone else, and him donating to a cause which denies people their rights seriously calls into question his judgement and whether he is fit to be the leader of an organization which espouses liberal ideals.
I'll give you another analogy. If Prop 8 were to deny the rights of black people getting married, and Eich had donated $1,000 to support it, would you still feel the same way?