Chris links and summarizes the slip opinion from the San Francisco judge who ruled California's prohibition on same-sex marriage to be unconstitutional. I haven't read it yet, but it sounds like the judge wisely stuck to asserting that gender/sex-based classification was unequal (which it is), and stayed away from the plaintiffs' claims about liberty or privacy rights.
The trouble with such claims is that they have little legal footing, in the case of liberty, or a kind of anti-legal footing, with privacy. How can one have a privacy right to demand a public acknowledgment and governmental recognition of one's union? Marriage, as a legal institution, is inherently non-private from the moment the clerk registers the matter. Liberty is scarcely more fertile ground. Again, it may be an excellent reason why the government ought to butt out of one's sex life, but it's pretty dubious in explaining why the government must butt into one's relationship to recognize it legally and load it up with the rights and responsibilities of marriage.
I worry that the plaintiffs' lawyers have gotten sex and marriage muddled, so that the justifications appropriate to ending the legal prohibition on sodomy are now shoved into the briefs for same-sex marriage. Privacy and liberty are negative rights; they demand that the state leave one alone to pursue one's happiness. Equality, on the other hand, tends to be a quasi-positive right. I have no right to vote if the state doesn't choose to hold an election, but once it does, I have an equal right to vote. I have no right to public education if the state does not provide schools, but once a schooldoor opens I have an equal right to enter. And I have no right to legal recognition of my relationship if the state provides no such recognition to any relationship, but once that recognition is provided, I have an equal right to it regardless of my sex or that of the person I marry.