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?
As I recall, the high-SPF sunscreen in the Robocop spoof included a warning that the sunscreen itself was carcinogenic. This is more evidence that that the intent was not to warn of the very real risks of the sun but rather to take shots at the rapid increase in SPF ratings ( going from 2 and 4 being normal and weirdos buying 8 to 30 and 45 being common on the shelf if memory serves ).
Edit: sorry, it was 5000 SPF https://youtu.be/8oJzfmWO3CU
And then my grandfather died from skin cancer. You bet that as an adult, I am not exposing much of my skin to sun at all when I am out on a sunny day, and I apply sunscreen to my face daily. I have no issues with being shirtless otherwise, and do it quite often, mostly at the gym.
Disclaimer: my gym is a small local one, not a massive chain, and being shirtless there is both explicitly allowed and is very common. At any given point, about a quarter of the people there will be shirtless.
I think a good barometer is to look at how people dress their kids. I don't remember any kids in my youth wearing sunshirts at camp. And while my mom always sent us with sunblock, it wasnt mandatory. Lots of kids at my kids' camp come well protected from the sun.
[0] https://www.cdc.gov/obesity/data/childhood.html
[1] https://www.cdc.gov/nchs/data/hestat/obesity_child_15_16/obe...