Comparing someone's efforts with what they could have done is a way to deflate and discourage them. While I don't think Sergey will care about your attitude (or even be aware of it), I care.
It's not that Sergey can only ever make this one comment in his life, and he has to pick the very best use of it. He can support more than one cause - and perhaps the journey of a thousand miles starts with the ground under your feet?
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.
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.