For nuanced discussions like this, we need more than SFW/NSFW. I think the movie rating system is helpful.
G — Pure wholesome goodness. That includes affectionate romantic kissing of the kind you’ll find in these Disney movies. I have a problem with marking this content NSFW, or certainly outright banning it, but that is literally what RUP is proposing.
PG — Similar to G but with slightly more violence, scariness, and adult themes. Still oughta be SFW since this is a 13+ site.
PG-13 — A fuzzy category where NSFW markers could possibly apply, but still ought not to be banned for PRESIDENTS, IMO. The cartoon anime boobs you posted above seem to me to be PG-13, rather than R, since it seems to be for primarily humorous rather than sexual intent. PG-13 borderline content features in attack ads all the time, so cracking down on it could present major issues in terms of free speech.
R — This content (sexualized, gore, harsh curse words, etc.) should be marked NSFW anywhere on Imgflip, and could permissibly be banned here. Worth noting as well that racism, sexism, and homophobia could fall in the R category as well — so watch out, RUP, this is a double-edged sword. :)
NC-17 — Nudity, actual sex, etc. isn’t allowed anywhere on Imgflip so this is a non-issue.
Overall, it seems everyone understands PRESIDENTS isn’t the boobs or hentai stream. This moral panic looks like a solution in search of a problem. Because there’s such wild disagreement over it, I don’t think a rule on this can be codified but must be decided on a case-by-case basis.