Mathematically, the cleanest solution is for them to use a commonly observed randomizing device: they flip a coin. Heads it's football and tails it's opera. And once the coin lands, there's no incentive for one player to switch, as it would only result in the loving husband and wife going separate ways for the evening and the loss of all preference points.
Actually, there's another option. One person can get mad. Imagine that through her anger, she's able to remove two of her total preference points—she threatens that if they fight over the decision, she won't have as much fun no matter what they end up doing. Now the grid looks like this, with the original preference points on top and the points if she gets mad below them:
The outcome: it's best for both if the guy gives in and goes to the opera before she gets mad. Thus they are guaranteed five preference points (but the poor dude never gets to the football game).
And the decision comes down to this: the person who is understood to be naturally capable of burning the most preference points and most likely to burn them (i.e. can and will get the maddest) will earn their top-choice activity, every time.
Here's another interesting dilemma: To buy my new book Brain Candy or to let your brain gently slip into disrepair, eventually resulting in settling forever onto your couch like a barnacle to watch an ever-repeating version of the movie Beverley Hills Chihuahua. Really, the choice is yours.