Avatar Pics
Well, now I'm using the Naturist symbol that was discussed on another thread.
Before that, I used a picture of my face. It was clothed, because that's the way I look when I meet most people.
A lot of people prefer symbols, cartoon characters, and the like to both hide their identity and yet display some facet of their personality which isn't evident with just a face shot.
As for men putting pictures of nude women in their avatar, that's just weird to me, unless they're trying to say that they have a female side to their personality. In that case, I get the message.
When looking to various groups I wonder why quite a good numbers of avatars do show Comic pics, or animals or alike. Also why do mere male profiles show third party female pics? Why do apparent nudists show themselves dressed?
First off, I think the male profiles with third-party female pics (women they obviously do not know or have permission to have on their profile) is a major red flag and you should not associate with them.
As for the other part of your question, most legitimate profiles have reasons behind their profile picture and the other imagery on their profile. Many like myself have privacy in mind. Though it is often a longshot, they do not want someone they know accessing nude pictures of them. I myself work in education and it is a definite risk to my job if someone were to find images of me from my profile. Frequently, other profiles on TN ask for more pictures from me (mostly strangers hiding under the excuse "to get to know me better"). I tell them my situation and the risks involved if I were to post an obvious picture of myself (nude or not). I also tell them that those who know me or met me at a resort or club know it is me through our conversations. Many also toss me the excuse (which has become a bit of a pet peeve for me), "If they see you on here, they won't turn you in because it reveals they were also on this website." That does not matter. Many people do not realize the double standards teachers face. I had a teacher I worked with get called in by our principal and had their job called for by some parents because they were seen out having drinks at a bar with their husband on a Saturday night. They were not drunk or behaving erratically, they were just out on a date. The parents who turned them in were also in the same bar drinking but that didn't matter.
The point I am making here is everyone's situation is different and do not just judge them based on what their profile picture is.