For most women it's something to avoided at all costs. Even now I think there's little worse than seeing a woman or¬†girl with a skirt or trousers on that are too tight and show the line of her knickers. But for a boy it's even worse.¬†
As a child I remember being made to wear a pair of my little sister's shorts. She's two years younger than me, so inevitably her clothes were too tight on me. She had a pair of shorts my mother sometimes made me wear. They were cotton, and not very stretchy. When I had a pair of Lucy's knickers on underneath the line of the frilly¬†trim around the legs would always be clearly visible.¬†
So if it hadn't been embarrassing enough to be made to wear the pink girly¬†shorts there'd be the added humiliation of everyone knowing I had knickers on underneath.