The problem isn't nudity, it's nudity in a public place where the nudity will be seen by people who haven't opted in to seeing it.
If you go to a nudist beach or a life drawing class or a sex club then you know that there will be exposed genitals and in deciding to go to that place, you're agreeing that you'll probably see some genitals. You've made that decision and you're not going to be offended by what you see.
In a public place where there's no indication that genitals will be on display, surprise, unsolicited genitals can be offensive or distressing.
There is also the context here of the history of men exposing themselves as a tactic to intimidate women. Whether it's sending unsolicited dick pics, having a wank in the park at walking-home-from-school time, any number of other incidents of these sorts...there's a nasty undercurrent of power play and intimidation in enough of these flashing incidents for it always to be a seedy and distasteful thing to do, even if it's not intended to cause distress.
When I was 18, I was sat in a takeaway with a friend, eating chips at the end of a night out, and a bloke walked over to us, flopped his dick out and placed it on our table. Should we have been okay with that? Was it "Victorian" of us to not be? Were the takeaway owners wrong to kick him out?