In England, we understand proper beach attire: but yeah, its generally fashion, they used to be in, now they're out. some people still like them, most don't.
They don't look very comfortable. Junk is smashed, and seams cut into any fat on the butt or hips. And the spandex stretches across the butt cheeks like a trampoline. Ew. Just as women wear fashions that are most flattering for their body type, so should men. Swimmers wear the for logical reasons, they stay in place in the water. And just about any swimmer looks good given their hard body.
Actually, with the proper fit they are quite comfortable, ur junk don't even notice. The uncomfortable thing for me are those loose, baggy bathing shorts that the boys wear. They are cumbersome while swimming, fill with air in the hot tub and when you get out cold water drips everywhere and is uncomfortable to me. There is another generality - women prefer boxers to briefs. IDK why - if you have a good physique, briefs look really nice on a guy. (If you don't have a good physique, boxers ain't gonna help)