The simple fact of the matter is this, the website already has a built in function for separating SFW and NSFW wallpapers, and even goes the extra step to add a 'Sketchy' listing for the monumental grey area between SFW/NSFW, something most sites do not, and will not ever do.
These features alone render any need for removal of 'overly sexual' images not only pointless but utterly redundant. If the concern is that all the nudity people may be exposed to if they make an account, verify the account, and then enable NSFW, even needing to change their ccount settings from default values in order to avoid having to re-enable NSFW content with every subsequent login, unfounded, and quite frankly ridiculous.
If people are believing this site is 'losing value' as a wallpaper hosting service because of the nudity, then just disable it's ability to be shown to you, don't arbitrarily try to force your views on others.
You're missing the important part how broad "NSFW" is. The site was slowly reaching the point where the obviously porn pictures became nearly inevitable if you had NSFW enabled. I might like wallpapers with tags like "lingerie", "bed" or "wet body" which do leave a fair chance they will be NSFW, but then you have no way of filtering out the pretty/classy ones from the full-on porn ones with spread legs and all.
Practically speaking, the only solution I would see to that is... a fourth category. But then you reach the common sense point: why do you need a "porn" category on a wallpaper website?
It's pretty simple and logical when you break it down to practical matters.
They do have a filter for that, and explained it pretty plainly, it's called 'Sketchy'.