He didn't just make a small donation once. He made a donation to a political campaign that called thousands of people sexual predators in campaign ads because it didn't actually have anything truthful to say in opposition to gay marriage. It was one of the most shameful and disgusting political campaigns in US history. And Eich made his donation after they started running those ads, so he was aware of how his money was going to be used.

And that was why it was an issue that he made the donation. Not because he opposed gay marriage (many people did and still do), but because he supported the wholesale slander of thousands of people.

If the Prop 8 campaign had been run differently, his support would not have been an issue.