Why do you, if you do, feel threatened by nudity, and the existence of naturist groups?

I find that those have never been to a nudist club or beach tend to equate sex with nudity much more than those who frequent nude beaches or belong to nudist clubs. I was talking to a friend of mine who said he gets much more excited by a woman wearing a bikini than a nude woman. The bikini adds to the enticement for him. What happens if it slips or moves or reveals...... A nude woman doesn't have any secrets that he wants to see more of.