Be the change you want to see in the world - Gandhi
People often see public figures as deities, and abdicate their own center of action to these gods. Saying "the government/corporations/rich should do something" strengthens their power and diminishes yours.
But they aren't gods. They aren't that powerful.
Sergey holds a special place in the world since he is one of the wealthiest ($15+ billion) and holds major control over one of the wealthiest companies in the world ($135+ billion - more than even Coca Cola). Most of us would say that places considerably more responsibility in his hands when it comes to taking on world problems. Just a sliver of his wealth can change the lives of millions.
So the letter really is an attempt at a wake-up call for Sergey. The homosexual marriage movement is wealthy, well-supported, and has a very loud megahorn. Using a public company to support a divisive political measure for already wealthy and vocal adults when there are children languishing elsewhere without a voice, just seems like a misplacement of Google's power.