Oh, come on now.
I don't believe for a moment that Barack Obama or Hillary Clinton "evolved." I don't think either one of them ever thought for a moment that allowing gay people to marry was any problem at all.
What they did think, and correctly, imo, is that at that time if they came out publicly in support of gay marriage, they would never be elected president.
So they both had to make a decision – stick to their principles and allow a republican to assume office (a Republican who would set back gay rights for a decade) or pretend to be in favor of civil unions, but not actual marriage, and keep their chance of being elected.
This is the type of choice politicians often have to make. You can call pandering if you like, you can call it dishonest, you can excoriate them for it.
Or, you can look at the actions Obama took to advance gay rights, actions which I believe Hillary would have also done, and judge them by the results they achieved rather than going down in flames for boldly proclaiming their true beliefs.
Then, they could have sat back and watched president McCain eviscerate gay rights, and smugly held themselves up as exemplars of great moral purity.
Talk is cheap. We all understand the hypocrisy of "do what I say, not what I do." You might also want to look at watch what I do, not what I say.