It would seem that the dik is a muscle like any other and needs use to stay strong. A guy who's getting none or one who loses interest in sex would therefore be at a higher risk of malfunction later no? Also suppose you're Dating or married to a fat smelly pig. Maybe the thought of some luscious boobies makes it easier for you to plow away. Not sure.
If you want the totally inaccurate theory, look here.