Greetings to the people, the question has arisen if, according to the plot, the main character communicates with a friend (no sex) or he has a mother / sister and he finds out that she is promiscuous and has sexual activity and on the basis of jealousy, mс also wants to join the fun as a result. Is it netorare or is it already netorase? And do you have similar games in mind?
If I understand correctly, you are asking which NTR category would best fit this particular scenario: Male MC finds out that his Mom / Sis / Female Friend is very promiscuous and gets jealous about it.
My 2 cents on the matter, and something I would like more people to accept, is that it is never a black and white matter and different writers could make it fit wherever they wanted.
Example 1 - Remove the jealousy aspect and it does not fit anywhere. As long as there is not any feeling of a relationship (of any kind) being broken then it should not be considered NTR at all even if the MC ends up watching somehow. Imagine MC walking on his friend / sister with their BF, without any negative emotions from MC this should not mean anything in the context of netorare or netorase.
Example 2 - MC finds out, gets jealous but there is nothing he can do about it, except be forced to live with it. This would fall into the netorare side of things, and even if he ends up watching as long as the MC's feelings towards the acts are of hatred then it would not break into netorase IMO.
Example 3 - MC finds out, gets jealous but tries to pursue that feeling more. He gets excited to watch, or even intentionally manipulates people and situations to push the girls into situations where due to their promiscuity MC knows something will happen. This scenario would be well into netorase territory.
Example 4 - MC finds out, gets jealous and tries to do anything in his power to prevent other people from approaching the girls, "protecting" them. Even if he himself doesn't have a chance as long as he prevents others I feel this could still somewhat land into the netori category.
Apologies since I did not follow the entire conversation from the past pages but yeah this would be my opinion on the question as I understood it.
TL;DR - it is not a simple black and white decision and it fully depends on the writing.