You kind of answer your own question here. Putting money behind an issue amplifies both your voice and the cause's well above and beyond the one-person-one-vote voice everyone gets (or is supposed to get). Everyone does not have an equal ability to contribute to political causes, so those with more ability have more power than those who don't.
And of course there's the fact that in modern democracy your vote is considered a part of your private life. Everyone has the right to keep that basic contribution to civic life out of their personal and business lives. But not for going above and beyond.
So it's really not that he held this political opinion, it is that he used his money to support it, almost certainly in full knowledge that it would be public information that he did so. And now, no matter how many times he says he believes in diversity without really explaining why he supported that cause if so, every gay person he works with knows that he spent $1000 supporting the cause to prevent them from getting married to someone they love. It would be kind of ridiculous if this did not cause friction. It's very difficult to "live and let live" when it's your life people are mucking with.