"Why does society in general condone the shaming of men who choose to date women who are much younger than them?"
Society in general condones the shaming of men for almost any reason imaginable. Lets get that out of the way right off the bat.
As for the specific situation you've outlined, I suspect men that participate in such shaming may, at their core, be jealous of male counterparts dating women much young (and I expect more attractive) than women their own age, while women who look down on such men could do so for a number of reasons.
Perhaps there's some bitterness amongst older women that they are being bypassed for a younger model, or perhaps they regard those men as using their age and resources to manipulate younger and financially settled women.
"Who cares if they are shallow? Is the sex consensual between two adults? Yes? Then let it be."
Fair point, but imagine if you were the parent of a daughter who, upon reaching adulthood, started dating a man your age or older.
I mean I'll be honest: if my niece introduced me to a boyfriend who was twice her age or more, it would be hard for me to not believe that she was being taken advantage of.