"horny jail" stream lol
Personally I don't see any problem with sexy images that are marked NSFW and posted to streams that announce their purpose as such (e.g. boobs, booty, MILF) so that those who don't wanna see it can avoid it.
Worth noting that Imgflip doesn't host "porn" -- i.e. pictures of bare genitalia or actual sex. The sex-themed content, jokes, etc. you'll find on Imgflip are PG-13 or at worst rated R. But anything NC-17 would violate site terms.
People who are posting NSFW stuff without marking it, or constantly posting in the wrong stream, or deliberately targeting other users with it, or otherwise sexually harassing them should be dealt with accordingly, and they are.
The status quo I feel is the appropriate compromise for a site with a large number of teenage users -- and even younger, although per site terms you should be at least 13 to use the site -- and also plenty of 17+ adults.