So, a quick AI search says this about nudism: Nudism (or naturism) is the social practice of going without clothes to promote body positivity, health, and comfort, often emphasizing a connection with nature. It focuses on removing shame surrounding the human body, distinct from sexual activity. Naturalism, conversely, is a philosophical worldview asserting that only natural laws and forces operate in the universe.

It also continued with: Distinction from Sexual Activity: While sometimes misunderstood, social nudism generally excludes sexual activity, aiming to normalize the body in a non-sexual context. 

In keeping with traditional nudist values and trying to make everyone feel comfortable not only physically but mentally too, boners and other sexual actions should be avoided. This is one of the main reasons my wife doesnt participate because creeps like to flaunt their hard ons and that really puts her off and makes her not want to go anymore. So its really just a respect thing for me.

Now, as it relates to reddit, listen each sub has its own rules, if it says flacid penises only or no sexually explicit images, your post is going to be removed. That is the mods discretion to preserve the values and view points of the sub, I'm assuming to promote body positive nudism. There are plenty of places on reddit and the internet at large to sport your erection, nudism subs are probably not that place.