Really, the thing that matters is that the Netori (the one stealing) has to completely take the affection or loyalty of the betraying partner away from the "victim". My words are a little more moralized than some people like, but that's the basic premise. Note, I say or loyalty, because not all NTR involves a romantic relationship. Your best friend can be stolen by a netori, your mom can be stolen by a netori, your sister can be stolen by a netori, the latter two, with no incest involved.
When a game has optional NTR one of two things has happened. In the first option, the dev has made it so that if you don't make certain choices, the Netori will never come into play in the first place, or there is an option in the game to turn it off completely. An example of this is My Girlfriend's Amnesia, in which it has a menu option. I haven't played enough NTR games to point out one where making the wrong choices brings a Netori into your life, but suffice it to say, in these cases, once the Netori is there it's unavoidable.
In the other option, the fact that F95 is trying so hard to keep the Tags down to a manageable amount causes certain games that don't have what I described above at all. But, it has cheating or adultry or other scenarios wherein the heroine might have sex with someone other than the MC. A good example of this is Babysitter, in which, if you DON'T stop these things from happening, either by not pursuing Christine, or worse, by pushing her toward them, she might end up being courted by either Robert or Silver. In this game, because of the way that these situations are presented, it's always your fault if she ends up with them. For Silver you have to not warn her, and then not stay with her at the hotel to protect her, and then still not be there for her. For Robert, you have to not dance with her, then let Robert into your house, and then not pursue her. All of these are optional, and if you take those options you don't get to claim NTR, because you never had her loyalty to begin with.