Okay, so disregarding that label, most platforms will have a target or focus or niche (well, the ones that want a chance of surviving, anyway) and they will thus be very wise to tailor the rules around that, and create the conditions ideal for fostering that type of content.
For instance, if you were starting, say, a TikTok-esque video app but for super-quick tutorial videos, wouldn't it make sense for upload criteria to require it be some sort of tutorial, stay within some time limit, and probably not be just a gratuitous video of a bunch of people having sex? Call it whatever you want -- "NSFW" is just a shortcut, a heuristic that most people understand the meaning of regardless of whether it is actually safe or unsafe at their place of work. But there can be no denying that platforms/communities serving some interest or demographic will have their own unique requirements, their policies and standards will reflect this, and very often this will preclude "NSFW" content.
A lot of people get bent out of shape about this and view these sorts of policies solely as some sort of censorship issue, but many fail to realize that most of the time it's just about creating the ideal conditions for the community/platform to take hold.
Isn’t this the opposite of “listen to your users”? If you built a platform and you think it’s for x, and your users use it to do y, isn’t y what you should be focusing on, in the sense of Lean Startup/Build Something People Want?
Just because you are legally entitled to do something doesn't mean it's good for your users or society: e.g. the widespread practice of IAPs, or Apple's censorship of the App Store.
NSFW is a euphemism. Please don't push your American puritanical worldview on every user of your UGC platform.