What has to be considered is that there are over 11000 members on this forum, policing them would be impossible without other users reporting them. Someone could easily have a NSFW avatar and not the first post it's on is the one that you look at with the boss nosing in.
I agree that avatars should be policed, but I don't see how would be feasible either. I don't think at the moment it is a great problem, the worst i've seen is something i'd see no worse than on the front on FHM, provocative but not really in the boundaries of NSFW.