He wasn't forced to resign, and he didn't peacefully oppose. The company, who's main product relies on a userbase to be able to use search engines which they are paid a (very, very small) amount to point towards Google by default. People were outaged at him and threatened to stop using the product.

Now they weren't outraged because he opposed gay marriage, but because he actively donated to a proposition to limit the rights of people. That was a political act, not a political belief, that relied on discrimination. Having a person like that as a head of a public company is not good, especially because it calls into question if he can reasonably separate his work from his personal biases. Would he not hire someone for being gay? Do we know if he have other discriminatory views? How can you know? And as a company, if you are paying a man, and he is using his own money to vote against gay marriage, you're indirectly doing the same as the company. Your name is attached to him.

Now on top of all this, he well and truly proved he had no right leading this company. He could have very easily lied and put this all to rest. He could have apologized for donated to the cause, said 2008 was a very long time ago, and made a simple sign of good will; but instead, he showed that his political acts and bigoted beliefs were far more important to him than leading the company. It simply didn't mean that much to him clearly. So he can fuck right off and go talk about the dangers of gays all he likes now, if he so pleases.