The term "political issue" doesn't really make sense to me, honestly. A political issue isn't really "political" so much as it's "currently in contention"; Catholic-Protestant civil wars in Europe were political as much as they were religious, because they had political impact. Gay marriage is a political issue because, and only because, it's under contention.
Thus, every issue is something a special interest might decide to take a stance on whether it's Google or a trucking company. The question is whether or not it's relevant, and it's demonstrably relevant to Google, which has gay employees. (It would be okay for Exodus International to lobby against gay marriage, too. Which I expect they do.)
Is it be morally right? That depends, but public opinion in the US and in many (but obvs not all) other industrialized nations suggests that it is wrong.
Shareholders can and should vote with their feet if a company takes a position they don't like. Ostensibly that's one of the levers available to you in our glorious free market economy, amirite?
You might alienate religious people, but when I consider the merits of this particular topic, screw 'em. There's no reason their views (a choice) should trump other people's basic human rights.
Of course morals come into play as well, personally I'd never run, work for or invest in a company that did that, and I like to think I'd still have that stance if I wasn't gay (actually I don't need to say "like to think", as I already have that stance for various other things that don't directly affect me).
What's next? Convince Japanese and Indians to do away with arranged marriages? Where do we draw the line?
Or are we trying to remake the world in our image? That strikes me as very dangerous.
Personally, it bothers me more that there's a MacDonald's in every street corner than Google's campaign, even though the former isn't intended to promote any cultural values.
But you haven't seen the horrible excuse for American food that is Pizza Hut internationally...... I swear after you see the Cheeseburger Pizza and the Chicken Nugget Pizza you will never complain about MacDonalds again....
But let's suppose truckers were overwhelmingly opposed to gay marriage, would it be ok for a trucking company to lobby against it? That would bother me just as much as a shareholder as Google's stance. (And more in my capacity as a citizen).