You can't talk about nudism without eventually bringing up the subject of sex. You just can't. Ironic though it is, this is at least as fundamentally true (and maybe more so) as the fact that nudism is not about sex. We should accept that. And if it's because a lot of outsiders mistake nudism for a sexual lifestyle, that's only because there is an aspect to nudity - not the whole thing, but an aspect - that is intrinsically sexual in nature. As long as we are sexual organisms biologically programmed to appraise each other's potential for mating based at least partially on visual stimuli, nudity is going to carry the potential for sexual stimulation. (This is perhaps more true in a textile society where nudity is taboo, and less frequently encountered outside of a sexual context, but even in a nudist utopia, people would still be attracted to each other's nude bodies, the way people in our society are still physically attracted to each other even while dressed). And while we may at some point in an advanced, transhuman future find a way to change this, I'm not even sure we necessarily ought to (because unless you're an anhedonic prude, this sort of stimulation is not at all undesirable). As it is, until then, you can't talk about nudism without eventually bringing up the subject of sex.
Now, being sex-positive in a sex-negative culture introduces some challenges. Due to our culture, most people are sex-negative to some degree. Being sex-positive, I'm willing to confront sex-negative attitudes, but it's exhausting having to do it all the time. If I join a music group, people rarely talk about sex. If I join a sports group, people rarely talk about sex. If I join a cooking group, people rarely talk about sex. How many more examples do I need to provide? But if I join a nudist group, it's only a matter of time before the subject of sex comes up, and usually in judgmental tones. I'd be happy to just not talk about it, because that's not what the group is about, but it always happens, without fail. There seems to be this intrinsic antagonism between sex and nudism, because sex apparently hampers a lot of people's enjoyment of nudism in a way that it doesn't hamper the enjoyment of music, or sports, or cooking, or what have you. And that's because of this intrinsic connection between nudity and sex, that nudists just can't seem to accept. And while there ARE sex-positive nudists, they are viewed by the sex-negative nudists as not simply having a difference of opinion, but actively obstructing the principles of the lifestyle.
I don't agree with this. And I don't mind people having differences of opinion, but being a nudist means that I have to be constantly bombarded by people's sex-negative opinions. And it's exhausting. Now, regardless of where you fall on the spectrum of beliefs on nudism and sex, is nudism a lifestyle that seeks to exclude portions of the population, selecting a special, enlightened populace to participate in a secret society? Or is nudism for every body? Agreeing with the basic tenet that nudism is not about sex does NOT require you to have certain judgmental opinions on sex in general - such as that voyeurs and exhibitionists deserve to be looked down upon, that nude and erotic photography is all "pornography" if it's not 100% asexual, that people looking at naked bodies and having feelings of physical attraction is a perversion of nature, or even that wearing clothes that emphasize sex appeal is vulgar and indecent. As long as you understand what nudism is and what it is not, you don't have to hold any of these opinions; you can enjoy sex and sexual expression without shame, and still be 100% loyal to the cause of nudism. But you're going to have to put up with a lot of guff.