To be fair, politicians (especially those who are running against a media darling) are asked "out in left field" questions all the time. The safest thing to say is "I don't recall" until you can regroup mentally and figure out what the question is actually about.
If Romney firmly denied this, then it turns out something did happen that was a little bit, sorta like the story (but not really), and he hadn't remembered it, then he'll look like a fool, a liar, or both.
I hear you and I think your view makes some sense.
But, what would you expect him to say if he was asked whether or not he raped a girl named Joan or molested a 6 year old boy? “I just don’t recall that”?
“Sir, there’s a report that you held up a liquor store when you were 17. Would you like to comment?”. “Well, I just can’t recall that, but if I did, I’m very sorry.”
There are times when you need to say, “No, that never happened.” That’s what you must say, if it never happened.
But, I do appreciate your point.