I've always been deeply confused by Christianity's hypocritical and duplicitous attitudes regarding the human body.
On the one hand, we're supposed to be made in God's image, and everything that God made is supposed to be good. On the other hand, it's supposedly wicked and sinful for anyone other than your lawfully wedded spouse to gaze upon and admire your uncovered form.
How in God's name ( no pun intended ) can you say that something is GOOD, and then insist that it be kept hidden from everyone? What the FUCK are we doing???
There's a lot to unpack with this question. You need to remember that Christianity started as an offshoot of Judaism and while Roman and Greek mores of the time found nothing really wrong with nudity, the Jews of the time DID have a big problem with it.
Now the 'made in God's image' refers to the soul/spirit vs the body. Further, Paul wrote extensively in his letters about overcoming our 'animal nature' and aspiring to the 'higher gifts'. Paul was very anti-sex and there's a heavy Pauline influence in contemporary Christianity, which I find to be extremely ironic given Paul/Saul's association with the early church.
All of which is off topic for the OP, but since we went to nude swimming, yet again, let me just say the 2 main drivers are 1) improvements in swimsuit fabric and pool filters and 2) Title 9.
Re the former: Who remembers swimming in cutoffs? Remember the fringe? Trashed pool filters of the time. Regular bathingsuits were cotton as well and just as bad. The fibers would wear and come off and clog the filter. Same reason women used to be forced to wear bathing caps. Long hair clogging filters. Lycra doesn't wear like cotton and it doesn't matter anyway as modern filters can handle a lot more than filters from the 70's and earlier.
As to the latter: Men may have swum naked, but women didn't. Once Title 9 required schools to have the same programs for girls as for boys it became easier and cheaper to do co-ed swimming and that, coupled with improved filtration, killed the reason for nude swimming.
It was never overtly sexual. I'm sure there was plenty of covert sexuality going on, but it was always about dollars vs sex.