Maybe, but it would not have stuck if passed unilaterally. When we collectively decided to legalize gay marriage, it was very clear there was a majority in favor. This came about by pockets of the government trying it out first.

You're overlooking the fact that people and politicians need time to grow and hone their skills. Without states, without giving them some ability to be corrupt, there is no testing ground. Politicians at the national level would not be experienced negotiators because the first step in being a politician would be joining the federal government. And I don't think forcing gay marriage on the US would've been a good idea any sooner even if we'd had a sufficiently progressive president and congress. I have deep sympathy for all the people who had to wait so long for society to officially accept them, but I think even they can respect that we might not be as strong of a unified society if gay marriage had become legal in, say, the 80s, rather than today. A unilateral decision by a president and majority congress could have set the issue back even further.