There's a story behind this. One day Larry goes to the amnor to visit his father. He less know that Burns and Smithers actually have a relationship (but I don't think that he can catch it anyway).
When he are about to enter to Burns' office, Larry see him and Smithers talking and laughing; when suddenly Monty grab Smithers' ass, making him blush furiously but then giggle for it. Larry thought that it's a kind of game.
After he found Ryan in one of the place they usually hang on together, then he grab Ryan's ass just for fun, but the young Smithers react to different from his father.
He blush, get angry, grabbing himself and then kick Larry's ass, who don't understand why he reacted in that way.
Ryan: WHY YOU...?!
He must apologize later and learning that anything that Burns and Smithers are doing, it's not a good idea making the same thing with Ryan.
Larry Burns (c) Matt Groening
Ryan Smithers (c) Me