On dickpic senders and flashers -
I agree with Donkey and thebewilderness - it is about the power, shock, revulsion (of the recipient/victim) that is tied up in the sexual thrill (for the perp only). It is primary reason they do it.
Also, the "peepers" (voyeurs), rather similar, in that it is the thrill of the (stealth) boundary violation that gets them off. It is all about the boundary violation in all of these cases, even though different methods of achieving it (one obvious 'in your face', the other stealth). And this is why many do progress to the 'ultimate' boundary violations - rape and/or murder. Power, control, boundary violation.
And Voice's observation of:
I don't think it's possible to be a naturist and an exhibitionist.
No, it's not.