Why is there such disapproval of big age differences?
Motdakasha said it quite succinctly (she seems to do that a lot on the boards
) but I'll give it a go.
Assuming that they are of the same general background, people of roughly the same age have much in common because they've experienced the same cultural/historical events, trends, technological advances, music, etc.
We are many things, but our shared experiences, our *touchstones* are very important. They are a shorthand way of knowing one another.
A couple with a twenty-year age spread, can in no way have the same shared life experiences and therefore their pairing, besides being biologically illogical, makes no sense culturally.
When you combine these reasons, plus the notion, whether it is valid or not, that the couple may be expressing maladaptive emotional needs, collectively you have a big negative societal reaction.