I read it as "Not safe for work". For other acronyms, I read them like they spell eg C-I-A or F-B-I. I always lose this fight because people can point to "official English usage" documentation. Its just that to my ears (and internal voice) saying "an N..." vs "a N..." sounds wrong. You're adding an extra "n" sound, so that when its spoken you say something like "aaaahhhnnnnnnnnn" with no break there's a loss of distinction in what's being said. Saying "A NSFW tag" where the letters are pronounced individually forces the distinction audibly.
See, you have a good example of where I'm wrong. I most definitely say "an NSA" in this case, because that sounds right. I think its just a few particular words that sound funny that way, while pretty much every other time the actual rules make sense.