So you are saying it isn't possible for relationship to decrease without cheating?
I haven't played this game in years but it is one of my favorites!
The point system is related to NTR turned on or off... after a few weeks and if you have started working for the gang you become a member with your step dad and David... every day or week (I don't remember well) your relationship points go down on all your girlfriends... and when the points are low the girls start fucking other guys behind your back...
If you turn off NTR your relationship points will never go down... so no cheating = no NTR...
Turning off NTR is not a good idea if you want to see the super hot sex scenes that are in this game!
With NTR ON you can get all the scenes, including those without NTR... so by turning NTR off you lose a lot of content.