This seems to be a creeping thing. In the 1960s and 70s, when I was young, all men and boys here (UK) wore speedos - or, as we called them then, 'bathing trunks'. Older ones were made of natural fibres, newer ones of artificial ones - including lycra. There was no suggestion that anything else could be worn. The only difference was that in the later 70s, square-cuts came in. Otherwise, they were the standard briefs- pattern - some briefer than others! LOL
I noticed that shorts started to come in during the 80s, and thought it was merely a fashion thing: why would anyone what to wear all that extra material while swimming? Then I picked up a sub-text, that speedos were seen as 'gay'. WHAT???
There is also a 'modesty' thing here, as other posters have suggested: shorts hide what one has - whatever size it is. So, the other night, I was not totally surprised to see a presenter on a peak-time TV programme wearing a wet-suit - and board shorts over it! Even the minimal bulge visible in a wet suit is no no longer permissible. And yet - it is perfectly acceptable for Beckham to appear flaunting his all in an underwear ad.