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.

One curious feature of the NSFW tag: it refers almost always to (a) images of nudity, (b) images or sounds of sex, or (c) swearing, typically involving one or more of George Carlin's seven words you still can't say on television. People rarely use the NSFW tag for images of extreme violence: instead, they're likely to eschew shorthand and spell out exactly what the potentially troublesome material is. ("Warning: these images are graphically violent." Or, "these images show the bodies of dead people and animals.") And in those cases they often, by adding a reference to "sensitivity," acknowledge that viewers could be upset by seeing such images.

The NSFW tag never does that, which is noteworthy. The implication of NSFW is that you, my reader, won't be bothered by anything you see or hear if you follow the link, but someone else might be: a co-worker, a client, a customer, or just the person sitting next to you at Starbucks. After all, the initialism isn't NSFAV (Not Suitable For All Viewers). But of course, if any of those people could be sensitive enough to be bothered by images of sex or nudity, or morally offended by them, then you, my reader, could be also.

I might not want to suggest that, though, because the broad online culture - like the still broader media culture it's a part of - doesn't have much room for sensitivity or scrupulousness about the representation of sexuality, which means that few people will admit to such sensitivity or scrupulosity. Those feelings are, to many, indistinguishable from prudery, and who wants to be a prude? In some circles it's one of the worst names a person can be called. So I'll assume that's not one of the options, and simply write: NSFW.

But here's the beauty of the NSFW tag: if you are such a sensitive or morally scrupulous person, you'll know to avoid clicking on the associated link. Whether you're in your cubicle or at the public library or at home all by your lonesome, NSFW is a highly informative and therefore highly useful tag, even if it does avoid recognizing the possibility of moral offense. On the Internet, no one has to know you're a prude.