Its the usual societal double standards for men and women. Women are considered $lutty if they have several partners, while men are considered to be gaining experience and sowing wild oats. Times are changing, hang in there.
My guess is that an older man and a younger woman can still have children together. Now with an older woman and a younger man, he just might be giving up his chance to have a family of his own. Now before I get jumped on, (ladies) this is my GUESS as to why SOCIETY frowns on this match. I am well aware of adoption options and the strives in the medical community.