very true! this phenomenon it's interesting. I think, at the pool, men could (not often) wear swim briefs or swim trunks and women wear one-piece swimsuits because many had worn them in swimming classes and they are used to wear this type of swimwear in the pool. Pools are, in most cases, related to sport activities. so other men and women wear sport suits in swimming pools. I also think the same: wearing swim briefs at the pool is more confortable.
but at the beach it's a different story: living in a "copy and paste" society and being contantly bombarded with advertisments in the summer promoting the beach and girls in bikinis and "masculine" guys in board shorts, I see that people choose to follow this beach culture imposed by the ads, their friends, the fashion trends, etc
well, this is my humble opinion
