Let's say there's an adult man from a western country having no extraterritorial jurisdiction and does an act in an African country that would be abhorrent and obviously illegal accordint to the laws of his country but it's legal in the country he's doing the act in. According to my logic he wouldn't meet chargen in none of the two countries because, as mentioned;
-the country he's from doesn't have extraterritorial jurisdiction
-the country he does the act in doesn't mandate penalty or prosecution for the former act
so, as far as my layman international law knowledge is concerned (yup, it's pretty little), there's no common intersection between the two countries' laws for which the man can be charged and prosecuted, hence he could even show a video of him doing the thing in front of a police station, if the video shows clearly that the thing is done in that country.
Am I wrong here? Obviously, the question is hypothetical in nature and don't tell me "it depends upon the 2 countries", because I stated the conditions that leave no ambiguity or genericness in the situation above.