19 Nor converts to Nandrolone which is supressive of natural testosterone production. So the longer you are on your cycle the more test is supressed. This will equate to not being horny at all even after weeks go by. You will also experiance testicular shrinkage so even tho you get physically larger from workouts, supps, and proper diet. You still lack testicular fortitude.
I am in my 6th week of a cylce and can confirm the above, but I will also say that sexually everything works fine it just takes more than the wind blowing to get things started, on the other hand I have no drive to actively seek out and score one for the team.
These two side effects will reverse some time after your cycle, and ZMA will speed up the reverse.
As far as some ProHormones making you horny and some supressing your libido, it is due to the target hormone. Some convert to Testosterone which would make you horny all the time.
due more research....