There is a big difference between mild NSFW and full on porno. Suppose there is a News story with photo/video but it's a little bit graphic or violent. Nobody is jacking off to that. Maybe shouldn't view it at work, but in the library is fine.
Maybe it's a funny meme pic but there's a nip slip situation going on. No biggy; it should probably be tagged NSFW. Probably don't want it showing up at actual work. But I want to enable this kind of content away from work without a bunch of actual porn showing up in my feed.
There should be a porn tag. It's not the same as NSFW.
EDIT: The two main devs have done some amazing work here, but as I understand it they are totally booked for the foreseeable future. My rust chops aren't quite up to snuff (yet) and my frontend chops are non-existent, so it might be a quite while before I'm up to speed enough to make a meaningful contribution. In the meantime just thought I'd point out the issue.
Mastodon has a system of user-definable Content Warnings that hide the rest of the post and any media attached. Due to just how broad NSFW can be something like this may be a good idea. Maybe you're ok with nudity, but not porn. Maybe you're ok with porn but not (certain act/fetish/whatever). Maybe you're ok with violence, but not dead bodies. Lots to think about there. Defining all of these in advance or trying to come up with categories such that it is actually useful to everyone sounds like a path to madness and fetishes you never even knew existed.
These can also be used for things that aren't necessarily NSFW, but also uncomfortable topics such as "miscarriage", "cheating", or even just "spoilers for ". They're pretty versatile.
I think being able to block entire instances as users will solve this issue.