Because, as so eloquently put by someone a few months ago here on TFP, when women get older they should have the "class" to stop having sex, because they are ugly and their vaginas are shriveled and dry.
/sarcasm
Mainly, near as I can figure it's because old men like to be horn dogs and chase tail until they take their last breath, and women are supposed to be meek and accepting of their husbands screwing their young, hot secetaries. My Philosophy of Sex and Gender class eventually decided that it was just another remnant of the time when men ruled the earth.
Personally, I think that if he wants it and you want it, you do it and fuck the age difference
__________________
Sage knows our mythic history, King Arthur's and Sir Caradoc's
She answers hard acrostics, has a pretty taste for paradox
She quotes in elegiacs all the crimes of Heliogabalus
In conics she can floor peculiarities parabolous
-C'hi