The reason Speedos are stigmatised

[Torchwatch]

Posing requires affectation, placing the body to best advantage for the spectator. A man can wear speedos, bikini briefs and even swim thongs without posing. One should act indifferently to any viewers, get on with what you are doing and ignore the looks of those around you. Sunbathe or run on the beach, swim in the sea going out past those that paddle in the shallows but do it for yourself.
Modesty is not in what you wear but in how you wear it. The muscly but fat guy in his board shorts that always seems to be in the way making other's step aside as he walks by kicking sand about is posing, showing how tough he is. The group of younger guys in speedos kicking a ball about are doing it for their own pleasure, of course people are watching but they don't care!