In Germany and the German speaking part of Switzerland nude bathing, saunas etc are common. There's a "beach" on the Rhine in Basel where you can sunbathe in the nude if you wish. It's quite popular. Although not easily visible from the bank as you walk past, it's very visible when you're in the river, which we often are in the summer.

So ... it's a cultural difference where many people just don't see their naked body as something to be kept private. However, as a Brit, I've not used the naked beach and do look out of curiosity each time I swim/float past in the river.

As for this particular topic, I've never flown F so the dilemma hasn't occurred. I'd probably take the pyjamas as a souvenir and sleep in my jogging pants and loose fitting t-shirt that I boarded in. That's what I do in Y or CW. Unless there's a shower on board I can't find the point in changing.