Well there is the “full faith and credit clause in the constitution” that would apply to gay marriages from MA, NY, and CT. This would apply to all states of course, not just CA. More speculatively, the equal protection clause could be considered to legalize gay marriage all over the US. This was precisely the reasoning used in outlawing anti-miscegenation laws. It was also the reason Nevada divorces had to be recognized everywhere. Even when NY state, say, recognized adultery as the only grounds for divorce, you could just go to Reno, spend six weeks in a cheap rooming house, get divorced the next day and hightail back to NY, divorce in hand.
Here is another federal issue. Presumably gay marrieds from MA, NY, and CT will be able to file joint tax returns. It goes back quite a way, but once upon a time income splitting was available only to marrieds in community property states (CA and a few others in the southwest whose law was based on Spanish law). This gave them a significant financial advantage (few wives worked after all). This inequity was cured by extending income-splitting to all marrieds (and then to heads of households). So Americans could end up having to recognize gay couples in civil unions for taxation purposes independent of state laws. Alternatively, they could pass a constitutional amendments banning it everywhere. Then twenty years later they would have to pass a repeal amendment.