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.
It may be mental baggage from my previous career in submarines (where "symbol speaking" was highly frowned about), but I simply say it out: Not Safe For Work, instead of En-Ess-Eff-Doubleyou. In that context it would be (for me) A NSFW ..., but it seems the right answer is in the eye of the beholder. :)
You also need to cater for acronyms that are usually pronounced as words (a RADIUS setup, not an RADIUS setup), and acronyms that have an accepted pronunciation that isn't simply reading out the letters (a SQL statement, not an SQL statement; although that last depends on style guides and local convention, both Oracle and Microsoft prefer the "sequel" pronunciation to "ess-que-ell").