I had this friend once who was a kind of roommate to me during my university years. He was not renting with me but likes to hang out in my room nearly every other day as I have the room to myself while others share. Of course, I pay more of the rent too. Anyway, this friend of mine is the type who will never, ever wear an underwear for whatever reason. He is always changing his clothes by wearing his top first, either a shirt or a t-shirt, so it will cover his ass and balls while he put on his pants. For me who is an underwear wearer through and through it was a bit disconcerting and I always let him use the room after he took a bath.
Some girls that I know love it when their men do not wear any underwear. Some don't. For those who don't mind, it may be because it will be easier for them to gain access to the things that underwear is supposed to protect. For those who mind, I think in their mind it is more hygienic for the men to have something to separate their crown jewel with their pants. For me, it will be easier to control the damn thing which seems to have a mind of it's own especially when it sense something which may excite it. It is also more hygienic as men as much as women discharge unmentionables once in a while, whether voluntary or involuntary.
There are also certain school of thoughts which says that wearing an underwear when you are sleeping will stifle the development of your lovable 'pet'. During my secondary school years, the juniors were always teased if seen to be wearing a pajama in which their undies could be seen as school's endorsed pajamas were usually white in colour and are see-through. The seniors would give the junior a lesson in the importance of keeping their most important asset breathable when they sleep especially when they reached morning and the thing start to elongate itself.
Nowadays, as much as I like to wear some underwear to work, I do tend to go on commando when I am wearing a snug and fit jeans which will keep the boy down. Even if it suddenly feel the need to wake up, in that tight environment, it usually goes to sleep as the constricted area will not allow it to survive. Unless, it receive encouragement from it's owner or someone who is interested for it to be awake....