This thread is hilarious!
(And yes, this daily hate has C&Ped much of the first two pages - it was shared on FB by the Christian Naturist Fellowship [yes, there is such a thing])
I haven't been on MN for a couple of months it was sapping too much time and it has taken nudity for me to come back! 
Firstly, the OP said Indecent exposure is a thing - err, no it's not (assuming you're in the UK). It used to be, but was superseded by the 2003 Sexual Offenses Act, specifically section 66; which was written in collaboration with British Naturism to specifically exclude naturism and general nakedness.
In short, nakedness in public is perfectly legal*; so it goes without saying that nakedness in a changing room is not going to be an issue. If you don't like it, don't look.
Secondly, modesty is an action, not a state of dress.
If you were dressed head-to-toe in a full-length bin-bag, gyrating your hips, dry-humping anything that moves, and grabbing your breasts, then I would suggest you are not being modest. If you are fully naked, and going about your normal business, then you are being modest.
Thirdly, of course YABU to be horrified by people in changing rooms, whether they are naked or otherwise.
*To clarify, simple nudity (which includes just standing around chatting) is perfectly legal.
If you showing sexually motivated behaviours, or are behaving aggressively and/or intending to shock and harass people by flashing, then this is illegal.