Okay, try this:
Older man - younger woman: culturally accepted for centuries in both life and literature.
Older woman - younger man: not as common in life or literature.
Why?
Childbirth is frakking dangerous!
Even in modern societies with state-of-the-art medical care doctors take the sure bet surgical solution in a third to half of all deliveries. (The 50% rate comes in one-child-per couple China, unsurprisingly.)
Factor that in and you'll see why older male-younger woman pairings are more common than the converse: many young women never made it to "older woman" while many a male found themselves a widower with a squealing baby. With replacement brides being cheaper than nannies...
Times and living standards change but the mores stay with us long beyond the point where they make sense.