it's probably because of the size of the company and what they're risking by having an opinion on this non-business issue.
Exactly. Breaking it down, what do they gain? Possibly some good will of the more liberal users. But powerful religious figures could just as easily (and almost certainly will) turn it into something of a pr nightmare that will ultimately affect some of google's users.
Even Disney has gay events at their parks. It doesn't get more corporate/"family friendly" than that.