Imagine people who really consider monster sex as NTR. Like cuckolding is even possible with non-sentient sex-partner, lol. You know that cheating requires at least three sentient partners, who can:
1) consciously understand the fact they are being cheated on;
2) consciously understand the fact that they cheat on someone;
3) consciously understand the fact that they are paticipating in someone's cheating.
So, if there is a beast who cannot understand the fact that he partake in cheating, there is no NTR. Its almost like cheating on someone with a sex-toy.
And even more - if relationships are promiscuous from the get-go and no partner demand faithfulness from the other, cheating (and so NTR) becomes technically impossible.
Because cheating is breaking the rules of relationships. If action is within the rules, however radical it is, it cannot break them and therefore cannot be considered cheating. And you guessed it - main cast are promiscuous fuck-heads.
The funniest part by far, however, is that you guys are okay with Cap fucking several women (and by your logic cheating with each of them on each of them), but not when they fuck with what basically is a living sex-toy.
It tells so much about you guys, so fucking much.