Not only are the Massachusetts homosexual marriages legal and entitled to full status for claims of benefits in Massachusetts but they will endure within Massachusetts even if a Federal Amendment is passed by the country to limit a marriage to male and female. The marriages in San Francisco may well be thus recognized as California has no "anti" law prohibiting them.
The problems come from recognition of the marriage by other states and the federal governemnt (here I can include international states as well). Each sovereignty controls marriage internally and can decide if foreign (to them) marriages are recognized. I would expect Canada and the Netherlands to fully recognize Massachusetts homosexual marriages while Saudia Arabia (and curretnly South Carolina) will not. Internationally, we are at the whim of the sovereign nation. Domestically, state recognition may be "forced" upon the state if and when a case reaches the US Surpreme Court and a decision is handed down that recognizes the right of one citizen to marry another citizen regardless of orientation. That decision will probably be based on the right of privacy and the recognition that orientation is included in that grouping. It is a veruy small step from de-criminalization of homosexual acts to the bonding that includes the expression of the bond by those acts.
At that point, the federal law that defines marriage (currently being legislated) may or may not fall depending on the character of the case presented to the courts. A victory for state recognition or for federal definition will be a powerfull precedent for the other. It depends on the words and intent of the case holding whether of not they both fall at once.
We can see that there is a high likelyhood of the courts upholding homosexual marriage. The decision in Massachusetts followed the precedents of the US Supreme Court and we can see the "knee-jerk" reaction from some members of Congress to foil the attempt (which is constitutional, by the way, but only used once previously in the Dred Scott decision) for such a case to reach the US Supreme Court.
Oh yeah - divorice will be the only way to end a legal marriage.