Anecdotally, I've noticed young men seem more insecure about taking their shirts off in public, even in contexts like at the beach, than they were in my generation (a couple decades ago). If you think your body is some gross thing that others shouldn't be subjected to, and that nudity is extremely awkward and embarrassing, I imagine sex is rather anxiety-inducing.
How do you know it's due to insecurity and not growing awareness of the dangers of ultraviolet radiation?
[0] https://www.cdc.gov/obesity/data/childhood.html
[1] https://www.cdc.gov/nchs/data/hestat/obesity_child_15_16/obe...