Because it normalises people being naked in public for no sensible reason. “Ha ha ha harmless naked people having a lark for a good cause.”
Children find it difficult to discern when adult behaviour crosses a line into sexualised behaviour. Adults who groom children are clever at exploiting opportunities to make nakedness seem innocent and amusing - situations escalate and it becomes dark and unpleasant.
If we are publicly promoting nakedness we create more of those opportunities for adults to say “well this is fine isn’t it, you know there are people doing all sorts of fun things with no clothes on, so it’s fine for us to be naked together.”