NSFW is definitely a sliding (multi-dimensional) scale, and different cultures and work places place themselves differently on it. At a previous workplace (software department in a manufacturing company in Western Europe) a women with only the lower half of her bikini laying on a beach was fine as a desktop background as long as you couldn't see her nipples. At a different company in field service exposed frontal breasts were still SFW. Meanwhile in many other jobs you would get in trouble for far less.