If more than a few minutes of screen-time of a feature film were similar to a given image or video clip, would that film receive an NC-17 (United States) or equivalent (non-US) rating based on sexual content or sexual content in combination with other content (e.g. sexual violence, etc.).
For a video longer than about an hour, would the video as a whole receive an NC-17 or equivalent rating based on sexual content or sexual content in combination with other content (e.g. sexual violence, etc.)?
If the answer is "yes" then it's almost certainly porn in the legal sense of the word.
If the answer is "no" then it may or may not be "porn" in the legal sense of the word but IMHO it is deserving of "free speech" protection in countries with "free speech" protections as strong as those in the United States.
One modification that would apply in "non porn" sexually suggestive images of minors or which appeared to be minors:
If the actors or characters in the film are believed by the rating agency to be underage (18 in the US) or they appeared to be underage (or the ages were ambiguous), then modify the above to be "if the movie was re-shot so the actors and characters were believed to be of legal age and they appeared to be of legal age" to remove the situation where a given scene would be "rated R" if it had adult actors and characters but "NC-17" if the actors or characters were either minors or their status as adults was not clear.
All of the above applies to live-action shots. It's my understanding that in the United States at least, the Supreme Court has ruled that non-obscene hand-drawn and computer-drawn imagery which does not rely on an actual child being filmed is outside the scope of "child pornography" laws because it is protected as "free speech."