Well my friend, this is a HIGHLY debated topic that tends to be very controversial, I can give you my two cents on it, but many people would probably not agree with everything I'm going to say.
NTR can basically be split into three categories, Netorare, Netorase, and Netori. These are all very unique from one other and Netorare is the one most people think about when they see "NTR", I personally don't consider Netori NTR at all, and Netorase is basically just a subgenre of NTR.
Netorare is to put it simply, you get cheated on and/or you're significant other/love interest gets taken away from you.
Netorase is basically Swinging/Sharing but with a fancy name and it encompasses both swinging and sharing, it's basically a
combination. You enjoy sharing your partner with others, and in some circumstances your partner is alright with sharing you.
Netori is the opposite of Netorare, you're the one doing the stealing this time, or at the very least sleeping with someone else's significant other/love interest.
Many people Hate Netorare and Netorase, these are very controversial tags, Netori in general is accepted by the community though. I personally despise Netorare myself, but am fine with Netorase generally, and REALLY enjoy Netori.
This game has both Netorare and Netori, as explained before, Netorare because if you neglect your GF (make certain choices) she will cheat on you with your teacher. There's also Netori because well, the whole game in literally centered around taking Nicole away from her husband. In my playthrough I'm still with Chloe and am on the romance path with Nicole, should be very interesting how that will turn out in the future.