It's not just Sergey's position. It's Google's position. And it is very related to Google, since there are a lot of gay software engineers, this is very important to many of them, and Google wants to attract and keep these employees.
Also, my sexuality SHOULD have nothing to do with my work, but if I were affected adversely by this or other similar discriminatory nonsense, it would most likely affect my work. I daresay the same would apply to anyone regardless of sexuality.
Prop 8 is about more than just marriage, it's about what comes with marriage.
I guess that this provides the cover for Google to discuss these issues in an official channel.
- Health care
- Immigration rights
So, yeah, it shouldn't matter, but it does.