I have talked to several non-nudists who said they would only go to a nude beach or resort to "check out the naked women". I think Nudism "not being about sex" is a reaction to the overwhelming number of people who have this mentality. Nudism isn't about gawking - it's about being comfortable in your own nudity.

Being that said - naked people are generally more attractive to look at than clothed people. Nudists understand that just because someone is naked, it doesn't mean they're offering sex. Naked people deserve the same respect and dignity that clothed people do.

As far as the switch you mention - there's much more going on with regard to human mating rites/rituals than just getting naked. Our animal brains know this even if we don't. I think the switch is instinctive.