That seems to be the case to me. I'm not completely familiar with US history, but don't you think the right of black people to be free and equal to the other people was already written in the constitution 250 years ago, for example? Or women's suffrage. It just took a while to actually recognize it.
Homosexuality was legalized in France in 1791 according to the same set of ideals, during the same movement that gave birth to the US constitution. Slavery was abolished as well, and women's suffrage was discussed, because it was well recognized that these were all questions of equality. It isn't a stretch to think that the US constitution would also fundamentally allow homosexuality and forbid discrimination according to sexual orientation (and therefore allow gay marriage as well).