From what i understand you seem to be going for a bit more realistic setting as in people including the girls have their own lives and will make their own choices. The choices of the player effect all the characters in the game not just the character you are interacting with.
About NTR some people like to divide it into several things but often confuse that with NTS which is different.
Netorare or cuckolding is having the lover or love interest stolen, This can be unknowingly (cheating) or knowingly.
Than there is netori which is stealing the lover of someone else this can also be done knowingly or unknowingly
In both these cases a lover is stolen.
NTS on the other hand is when lover or love interest sleeps with others with the knowledge and permission of the significant other. It's also known as sharing or swinging and no lover or love interest is stolen and it's also very common for both lovers to sleep with others.
When both lovers engage in things like treesomes, gangbangs or orgies it falls under NTS.
When no love is involved it technically doesn't fall under under any of these terms tho.
And this might send people with pitchforks storming my house but technically every harem is NTS since the girls are sharing a partner.