It's not what he thinks of gay people. It's that he spent his money to successfully deny gay people rights and nullify marriages that were already recognized. That's not somebody I want to give money to. Look on Hacker News for his comments about why he made that donation, and you might change your mind about whether he is actually a bigot. He has never apologized for his actions.
People have a right to support slavery as a government idea that is tied up in politics. /s