I appreciate that you have this point of view about the behavior of the characters and I hope you liked the game... I'm going to answer you the way I thought about the characters' behaviors
For starters, I've mentioned at other times that I don't plan for Tifa to stop being in a relationship with Cloud... I don't plan for them to stop being together (at least on Tifa's side) since personally that's not the idea of NTR that I like... I can't feel the NTR anymore if the two people are not in a relationship...
Tifa does love Cloud with all her heart, and Cloud loves Tifa with all his heart as well... I tried to show a lot of this by showing Tifa feeling worried about everything she does and also showing situations where she enjoys with her boyfriend... And sharing together as well, to make it very clear that they love each other and are in a relationship. The thing in this situation is that Tifa simply let herself be influenced a lot... She was satisfied with her sex life with Cloud, but simply from the beginning she felt attracted to Marvin in one way or another... And this added to the influence that Amber started to have, that together with Marvin did what was necessary for Tifa to fall little by little... Well, that's what changed everything because Tifa started to feel a different kind of sex, you know? I'm not saying it's better or worse than the one she had with her boyfriend, it was just different and she started to like it more... None of this would have happened if Tifa from the beginning hadn't had a predisposition for it to happen, clearly... She wanted it to happen and she let it happen, but it is true that a series of events formed that made her fall more and more... But she never stopped loving Cloud, as for example in different situations when she is in the middle of the act with Marvin or Barret, they mention Cloud and she doesn't want to talk about him too much, because what she does makes her feel very guilty even though she doesn't feel she can stop. She is slightly similar to Amber in certain aspects, as they are basically both doing the same thing, being unfaithful... The difference is that Amber is much perverse than Tifa, being that Amber really gets turned on by other big men fucking her behind her husband's back