Finally, you curtail your argument by saying “simple nudity”. Trust and Safety exists because of context and action. These arent tasteful nudes or art people are looking at.
I wont speculate too much, but the intent of a picture, the context and the target make a difference. Sending a pic to someone not interested in them, or a minor - has a vastly different connotation.
Heck, it isn't even universally linked to sex. I don't have a penis, but from what I understand, they do randomly become erect and cycle through this stage during sleep.
A penis, even in hand and suggestive, is still simple nudity. None of the people in the center have had it sent to them personally, and surely they know.
Harassment using pictures of yourself is different: That's harassment, and should be frowned upon. They are still using simple nudity to do it, and the nudity isn't the issue.
That being said, if it was a picture of a penis erect in sleep, the context of "probably not consensualy taken" on itself makes it more disturbing. And seeing a lot of these can easily make you more paranoid to sleep where you can't lock door and everything.
Spending time defining only the non problem parts is a luxury. Theres pages of que to clean and no one to do it. I need a definition, plus examples and tests that a team can actually use daily.
Furthermore - unsolicited sexual imagery IS traumatic to a swathe of people (survivors of abuse) and illegal in many cultures.
That moderators get affected seeing such content should not be a surprise. The very least that happens to mods is desensitization. Paranoia, PTSD, high risk behaviors are common fates if you work in tough ques.
In the context of a dating site, sending someone a picture of a steak is inappropriate and creepy. Is the user trying to communicate that they see someone as a piece of meat to be used for pleasure? Is it a threat ("I'm going to cut you up")?
Likewise, there's nothing inherently wrong with a picture of a penis. But seeing first hand the aggression and intent to humiliate or victimize that's behind a dick pic is what's disturbing. See enough of that every day and you'd lose your faith in men. That's the problem for the moderators.
If you had to view that harassment secondhand every single day, wouldn't you feel a little disgusted after a while?
Sure, you can look at it and say "this isn't about me", but does that make the disgust go away?