The Supreme Court of California has struck down marriage laws that limit marriage to people of opposite sexes, thus making same-sex marriage legal in America's richest state. I can't say whether the legal reasoning is sound until I actually find time to read the judgment. From what I've heard so far, it may turn on an expansive reading of constitutional "equal protection" provisions. This is an approach that I've never had a lot of time for from a strictly legal viewpoint, but such reasoning is now a feature of American constitutional law. In any event, the legal doctrines involved probably have little application outside of the US, which has built up a unique and massively history-dependent body of constitutional jurisprudence.
In all the circumstances, it's a good result for gays and for modern concepts of sexual liberty, but a better long-term outcome would be for Leviathan - the state - to get out of the marriage business entirely.
If people want to be married according to some religious or traditional idea of "marriage", fine. If they want to have some kind of ceremony to mark the occasion, great. If they invent some sort of non-traditional arrangement and want to call it a marriage (with a nice ceremony thrown in), I wish them well. I don't care whether they're the same sex, or different sexes, or how many of them there are, or whether the arrangement includes an intelligent squid from Alpha Centauri (as long as the squid understands and is willing). However they want to live their lives, I'll argue for their legal right to do so and I'll hope that they receive much love and support from their families, friends, and communities.
But the state should not be deciding what will or will not be given the social prestige that goes with "marriage". As long as all concerned are of appropriate maturity, let people simply engage in whatever sexual relationships and means of family formation they want. If they want to mingle financial assets and other contributions, or if they have kids, there needs to be a body of law to sort out disputes when relationships break down - but that's all perfectly practical. It doesn't even require a lot of additional regulation: if necessary, the courts are quite capable of applying well-established legal and equitable principles to get fair results.
What may be needed, especially in the US, is legal reform to ensure that people don't lose valuable quasi-public benefits, such as health insurance, by no longer being regarded by the state as in the privileged status of "married". Such issues might delay the practicality of what I'm advocating, but in principle the state should leave the entire marriage business to the individuals immediately concerned.