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.