If gay marriage had been a discussed issue back then, and he had campaigned against it in the name of defending traditional marriage, then I promise you that there would have been accusations of hypocrisy, and rightly so.

Obama says he is opposed to gay marriage while also saying that his feelings are uncertain. He does not campaign against legal recognition of gay relationships in the defense of "traditional marriage" and the children that apparently only straight married couples can raise properly. If he did campaign on these issues and then was revealed as having an affair, there would be calls of hypocrisy there as well.