Eich is Catholic, his faith does not believe in gay marriage. He donated to a charity that was against gay marriage on the ballots. Because of that he was forced to resign.
He donated to a charity that worked to take away my "god given" (I didn't choose to be born gay, I just was) rights. He absolutely should have been forced to resign and I'm glad he did. You're also a real asshole if you think that donating to an anti-gay charity is right.