“Boys and their mothers want pretty girls because a pretty girl equals a healthy girl, which means good genes to pass on to their offspring.”
Interesting theory, could be thats how evolutionists would explain why men developed attraction for pretty females.
However I really doubt thats why the boys mother would want a pretty girl.
Also most boys want skinny girls, skinniness is not a sign of healthiness.
If anything being a little plump is a sign of healthiness.