The mods are, themselves different from the image section, and are generally useless unless you're playing the game in question. The image share however has, almost since its creation essentially been a porn-share service that has been plagued with and endless stream of topless women in various poses. As the image share section can also be seen and used without the game running, and in some cases without even having claimed that you were over the age of 17/18, the rules had to change, both for the sake of the site, and for the sake of those who were using the image share for non-porn purposes, but who weren't being seen because they were quickly pushed back a few pages by someone uploading 15-20 nude images at once, of the same character, in roughly the same pose, just with a different angle and lighting.
The mod section of the site will likely never be restricted in such a manner since this would likely cause both legal issues as well as some rather aggressive outcry from the modders of this community for having their work behind some payed service. In this case, you are jumping to conclusions and totally missing the point. If somewhat explicit mods were seen as a real problem, they would (and some have) be removed from the site and have any mention of them be banned due to their content. But again, doing this on any large scale would cause more problems than it would solve since it would alienate many modders, including a few staff members, as well require considerable time and resources spent in trying to enforce such a policy.
But how is that really different? Owning Oblivion doesn't mean you are 18+ or older. I understand that the quality of most nude pics is mediocre at best, but there are many useless mods cluttering TESnexus as well (I'm speaking of the 500th "enables you to play as dremora" or the 1000th "lol i haz made god weaponzz" which require at most 5 minutes work in the CS).
As I said, I don't really understand the reasons. It seems it's a mix of "I don't want porn on my site", "I couldn't show the image section to my family or friends because of all that nudity/porn, would be to embarassing" and "Most of these pics are crap anyway". But on the other hand it's not really important if I understand your decision or not, It's neither my site nor do I have a say in that matter. But I do find it indeed kinda disturbing that explicit violence is allowed. When I was a kid myself violent images always had a greater (negative) impact on me than a boob or two. And those "ZOMG VIOLENCE!!11eleven" screens are on par with the crappy nude pics. But oh well, it's getting late and I should go to sleep.