Where to draw that line? I think I know how. Sex is on the minds of most all, much of the time. Thoughts come and go. There is generally healthily not much egoic control of them. Thinking can be disassociated from action.
How one dresses is not so much an activity. One can do about any activity dressed in about any fashion. Nudity is a style of dress. Sports, dance, work, or sex are activities that can have nothing to do with dress. Sex does not require nudity, any more than any other activity requires nudity. It is just a choice of dress, basically. It is simple nudity. Therefore nudity isn't about sex anymore than any other dress form.
Nudity can be disassociated from sex, or visa versa. To say that sex doesn't occur at all to a nudist is kinda silly. Thought happens. From my own experience and other's writings and comments, most people often tend to have wandering eyes at first in a socially nude situation, after coming from a textile world, but this drops away quickly. We will check each other out from time to time, and often be done with that quickly, apparently quicker than with clothes on. Thoughts happen, no matter what is worn.
A guy wanking, especially fantasizing focused about others, in public, brings others into his her sex. I like to say no to sex, or decide to participate. If someone is getting off, or trying to get me excited, it seems reasonable that I can avoid this manipulation and say no to the imposition. If someone imposed himself on DF, he may get his ass kicked, or interrupted in some uncomfortable way. These guys sitting in cars, not so discrete, or trying to be discrete but not succeeding, I try to ignore, or act as if I don't know, so as to not embarrass them. Maybe I should tell them that it is obvious. Maybe they don't know in their frenzied state of mind. It is extremely rare and I have better things to do than to interact with them.
Public sex is in the realm of public exhibitionism. Maybe it isn't such a weird thing, maybe it is, but it isn't synonymous with casual nudity. Sexual liberation, I suppose is for another forum. Body freedom, I believe is the matter, here.
Jbee