I'm not keen on courts' dictating what governments must say (as opposed to what they may do).
For one thing, we should avoid abridging the collective speech rights of the people or their legislatures. In my view, not only individual people, but also "the people," should be able to say what they want (even if it's wrong). The remedy to bad public speech is to rebut or criticize the majority's view, not to ask a court to strike it down. One can also simply ignore what the government says by, for example, calling gay people "married" even if the state won't.
Further, I think we should be careful about striking down state language or expression that treats citizens differently, even when the differences are invidious. That opens the door to all kinds of litigation about governmental expression. Do we want courts to decide the content of textbooks in public schools, the meaning of public statues and monuments, or statements made on the job by teachers and police officers? The state can do wrong by speaking offensively, but lawsuits are not the best remedy.
It's an interesting question in political philosophy whether states should be in the business of "speech" and symbolism. Staunch individualists (e.g. classical liberals) are more likely to suggest that the government should remain neutral on such symbolic/moral issues, offering civil unions to all and no official mention of "marriage" at all. Those of a more communitarian bent, on the other hand, hope that the state might give voice to 'the people' (as Peter puts it).
I lean more towards the liberal end of the spectrum. On a contentious issue like this, what right has a mere majority to claim the moral mantle of "the people" as a whole? Especially given the increasing ease of group formation, I would sooner see civil society, not government, as the platform for collective expression. Churches and other groups in civil society may bestow their blessings, and the label 'marriage', as they please. There's no need to co-opt the apparatus of the State to give voice to such groups. On the other hand, perhaps it could have good consequences -- encouraging deliberation and the formation of civic identity? (I'm skeptical, though. These emotional symbolic issues do not seem to bring out the best in people.)
What do you think?