For the purpose of this comm I consider art NSFW if it contains explicit sex acts or full nudity or is clearly meant to be porn even if you don't see genitals or sex acts depicted (like sexual fetish or bondage scenes or such). Also NSFW I consider extreme graphic depictions in art with disturbing content (like depictions of violence or injuries that have gore, especially if realistic and not cartoon styles, e.g. somebody realistically crushed by an anvil rather than a cartoon hit by one).

FWIW, when I have posted fanart myself I have considered two naked guys in bed cuddling and kissing with no visible genitals to be PG-13, though on the NSFW divide I got comments that considered it NSFW because of the US complex about nudity...

I try to gauge whether I would feel awkward to have the picture on my screen in a public library if someone walks by and looks at me looking at that.