Why would a man wear padded underwear? Well, like many of us males, you may not have much in terms of buttocks. No matter how nice your pants may be, if they are saggy and droopy at the back, they just won’t look good on you. If you want to fill out the seat of your pants, padding can help you out: it can round out your backside just enough but not too much. Some underwear comes with padding built-in, which is very convenient.
These seem to do the trick, but I’m no expert:
Padded underwear for men pictured: Leo Padded Boxer Brief, white
Of course, padding may have an effect on your overall comfort.