I feel overall that the SFW / Sketchy / NSFW categories and the uploading rules are quite clear and well-balanced. The only problematic part of the rule in question is the last item in the list:
"Sexual content (masturbation, oral sex, sex toys, penetration, bodily fluids, stretching/gaping of genitals), closeups of genitals, child pornography (including lolicon), and other overly sexual images are all strictly prohibited."
“Overly sexual images.” All of this discussion goes to show that what is or is not "overly sexual" is open for debate and comes down to matters of personal taste.
If that last clause were to be dropped, the rest of the rule (and the 5 images per set rule) would still be enough to prevent the site from becoming a “porn board,” while still allowing Wallhaven to be the freewheeling, ever interesting, and sometimes sexy place that it always has been.
I’d, say, if an image contribution follows the rest of the rules but is not to your personal taste, if you feel that it is too this, too that, too much, too nude, too explicit, has the wrong lighting, the wrong hip position, whatever, don't click on the image. Scroll on past it. Others, of different taste, may rather enjoy it!