Where does it come from?
1. Body shame. Boys and younger men today are ashamed of their bodies, sometimes because of their looks or build, sometimes just because they don't think they live up to an imaginary ideal standard. This hit its peak in the 90's with clothes that are so baggy you can't tell ANYTHING about the person wearing them. And now, skinny jeans, which look good on about 1% of the people that wear them, are exaggerating that ideal even further.
A corollary to this is helicopter parenting to prevent "discomfort" for Junior. I told my parents I didn't want to take showers in gym (in 1976 or something) and I basically got told to grow a pair (I was working on it but a really late bloomer) and ignore anyone who bugged me. End of discussion. I took showers.
Two days ago, my sister got into a big tizzy with her kids school, because her daughter didn't want to swim, she's "heavy" and doesn't want to wear a suit. She got excused from gym. Not just swimming, but all gym for the rest of the year. Which of course, is not the answer for a kid that's not physically fit to begin with.
2. Modern media imposing fear of pedophilia/rape/molestation on kids, so much so that the mere idea of an older person (and I'm not talking about "creepy old man" like me) looking at a younger person - for whatever reason - is considered a threat. Better hide your junk or someone might jump you...
3. The perception that boys getting undressed in front of each other might suddenly flip orientations and start kissing passionately..oh, wait, sorry, that's just my imagination.
4. The consistent swing to the right - morally, I mean - that the country has been taking for the last 20 years. We're becoming prudes because of a religious drive to move the country back to "family values" - homo is wrong, gay marriage is wrong, etc. - and being naked in the company of others is somehow wrong, as well.
/grump