Its not specific to nudity at beaches, but more of a general thing. Expectations about what a body should look like and the extreme focus on a binary gender is incredibly harmful for everyone, since it creates boxes were everyone tries to fit into. When we AS society collectively work on demolishing these boxes it remives a lot of stress for everyone who previously had been excluded by these boxes. Realizing, that bodies are different and that it is no shame to look different from some arbitrary beauty standard, that's unreachable for most people, is one of the best things that can happen to you.

You do occasionally see flaccid cocks in media and IRL, but only from guys confident enough to show it in the first place, which skews public perception a fair bit.

I went to school with a girl who'd only ever seen erections drawn. To her first boyfriend, she said "is that it?" the first time she saw a boner. It was a perfectly average cock, but she'd been expecting something the size of a cucumber

I live in an area of Germany where nudist beaches (FKK in Germany) are very popular, and it doesn't seem to help body dismorphia at all