What I consider should be labeled NSFW is anything you don't want a 7-10 year old kid to see, so a bikini is fine, but girls waring clothes "to tight", someone touching/acting inappropriately, really bad words, really bad gore, inappropriate covering like ribbons, a girl with a open jacket and nothing underneath, that one guy with that "mankini" thing, popular kid's characters talking/doing something bad (wielding a axe like a maniac), ect. Of course I have a Not Safe For Kids approach rather then NSFW, but there is a 99% probability that there are preteens geting on there, so just think about whether you want those kids to see the themes that is being put on the site.