I wear 501s most of the time without underwear. But I have a real soft spot for wearing white y-front briefs while hanging around the house, during foreplay and cuddling with a guy. I like the look. I like how you can 'aim' down, over, and up.
In 501s, briefs can bunch your junk up in a meaty bulge, and I like that look. But it sure binds after a while.
Boxers are cool in chinos, khakis, slacks, and also hanging around the house.
Briefs and boxers both are perfect for swimming at a pool party, unless it's swimsuit-optional. Then why bother.
My experience is only as a USA guy, my preferences and fetishes come from my life. Underwear in other countries are different from USA and create their own preferences and fetishes.