Patrikios If I read you correctly I agree with your position.
There is a lot of ambiguity in both "natural" and "necessary" and I think it is very ill advised to be overly dogmatic (in the worst sense of the word) as to what activities or what desires go in each category.
That's why I try to emphasize that the categorization is a "tool of analysis" (which is the way I see Torquatus describing it, as a conceptual framework toward classification but not a bright line) , rather than trying to use the classification as a stand-alone blueprint.
Ultimately I don't think there are bright lines involved here other than the question of whether the desire is unlimited. If you pursue unlimited amounts of something you are by definition going to fail because as a human being you are limited by time and space. That's a bright line, and it can include sex if you choose to pursue unlimited amounts/types of sex.
II added this note near the part you quoted:
NOTE: In case it is not clear already, in the following discussion all specific desires are contextual and relative to the time and place and persons involved. The qualifier GENERALLY should be understood as applying to all specific desires, except insofar as any desire is pursued in UNLIMITED fashion. Depending on context the classification of a particular desire may shift from category to category, but the pursuit of any desire in UNLIMITED fashion is always going to be definitionally unattainable.