well, there is a reason, lots of experience and of course certain wisdom behind a girl being younger in age to her life partner. The glue that holds a marriage together is its intimate part and of course herein lies the answer. Women tend to look older physically when compared to men of same age....... so, the intimacy begins to be affected as men tend to grow younger (mentally) with age.
Yes this is debatable, but not all relationships are ideal so it pretty much holds ; in all good probability, there would always exist a chance of such a marriage being affected in such a case'