Let me say for the record that I am not particularly enamored of this line of questioning, because I fail to see how it materially affects one's job performance as POTUS. There are several of our finest presidents who would not have held up to that scrutiny. However, since the Republicans crossed this threshold during the Clinton administration, making it an issue worth millions of taxpayer dollars to investigate and prosecute, it is only fair to hold them to the same standards.
Cliff Schecter:

(F)rom a town hall meeting in Nashville, Tennessee Monday, mixed in with platitudes about gay marriage, we get a nice little comment from this questioner on the sanctity of marriage in McCain's life--or more to the point, sanctimony. Here is a rough transcript of her question to The Morally Righteous One, which comes at the beginning of the video (it includes McCain's answer to this question and a previous on on Hillary Clinton):

My second and final question, you talk a lot about the character issue...and...like you, um, I was opposed to gay marriage, I was in always in favor of civil unions but the basic definition of marriage....but, then I get to thinking, that is based on what we consider to be the sanctity of marriage. There is nothing....you see long-term couples splitting up, it's, it's just crazy...I know that you, your own situation, you're going to have to address that in the campaign. Infidelity is just a terrible cancer on this country....and I think if we're going to talk about...gay marriage, it has to be in the context of the preservation of marriage...which I just don't see it, I think we need to make it more difficult for people to get married, or whatever we need to do..if that's...if we're going to be consistent.

McCain ignored that part of the question, of course.