How better to signal that content is objectionable without presuming anything about the individual you're warning?
If you want to be pedantic about it, NSFW isn't an acronym, because true acronyms (NASA, BAFTA, and the like) are pronounced as words, and you can't do that to NSFW without spraining your lips. NSFW is an initialism. But what's interesting about it is not what it is, but how it functions, which is as a tag that identifies certain very specific kinds of content. It does this job in some pretty odd and rather subtle ways that are worth unpacking.
When you tag an image or video or sound file as NSFW - Not Safe For Work - you really don't mean "work," you mean "any place where where your co-workers, clients, customers, or innocent bystanders can see or hear what you've got on your computer screen." Cubicle culture is its first context, but it's just as relevant in the coffeehouse and in some cases the living room. The NSFW tag is an act of courtesy from the linker. It says, "I don't want your boss to yell at you, or, worse, for a co-worker to report you for sexual harassment."
And it's not just courtesy, it's also a cue to empathy: a signal to the potentially insensitive that they should consider others' possible responses. Of course that's not going to have much effect on the kind of person who openly watches porn on the airplane, but for the majority of people, who are more likely to be forgetful than to be in-your-face with their entertainment preferences, it can be a genuinely helpful reminder.