My reason is simply that Im just too obscenely large to wear garments like that at beach, unless I want to be arrested for 'over exposure'. If i'm at a clothing required beach, I have to wear baggy trousers/sweats or exceptionally long shorts, with a towel over my legs when sitting down to prevent 'flop out'.
Thankfully at clothing optional/nude beaches I can let it all hang out.
Although this topic on g-strings reminded me of a time an ex told me about being in a store around Christmas time with a friend who suggested buying a novelty Christmas g-string, to which my ex partner told her friend that they wouldn't do any with a pouch big enough for me. At this point a shop assistant being nosy interjected, they did them in all sizes and what size did she require, she responded by saying 'one for a dick that's this big, XX inches', the shop assistant went red faced and slid away (they didnt do them in that size clearly).