Nice definitions but I think it's not that black and white.
I think that often, getting a third person into a relationship changes the relationship dynamics in a way that can be difficult to predict.
You see that in the Dee - Martin relationship as well. It started out as friendship between F and Martin, IIRC they did a threesome with some random girl before, a long time ago. Initially Martin was just a fitness trainer to D, but (depending on the route the player chose) some feelings already started to develop. The first F-D-Martin threesome was mostly F and Martin reliving their college days or something. Just fun experimentation, no strings attached. However in the last update(s), it shows Dee developing real feelings for Martin, which may well turn into "real" NTR.
In my opinion, it's a direction that F, both in his role as father and in his role as lover, should not be happy about. As a father, he has to witness Martin degrading Dee. As a lover, Dee seems to be falling for Martin and perhaps replacing F as the next new toy.
Finally, when it comes to the ultimate corruption, it may well end up with Dee getting gangraped for money by Martin's friends...
That's why I am much more comfortable with the Dee / Elena + Jennifer or Georgina routes. The Martin route is just a train wreck in motion.