You get emotionally attached to characters. Rarely do I see people rooting for the downfall and unhappiness of the main character especially in the romcom genre (yes, I know there are exceptions). But in romcoms especially the whole point of the romcom genre is to see 2 mc's (fmc and mmc) get together. The whole point is to get emotionally attached to both characters and to watch and see how they grow and evolve over the course of the series. You watch them fight, argue, laugh, have fun, goof off and love each other with the end goal in mind that all this build up will end with the pay off of when the 2 finally get together and and thr real fluffy wholesome stuff can commence. Now the problem with NTR is that it takes everything above and throws it in the trash. The connection between characters? Trash. Their feelings and yours that you got attached to? Trash. The whole end goal of these two getting together that you've been patiently waiting for and spending your time reading? Trash. Basically, if the story doesn't let you know it's NTR from the off then it's readers will feel like and will have actually wasted thier time. Also you know society and the majority of people see relationships as between 2 people (yes again, I know, there are exceptions) so when one character betrays and hurts another so deeply it's despised by the readers. Now if you can't understand anything I've written above then I just think you're brains wired different I'm not sure you're ever going to truly understand. But hey no 2 minds are ever wired the complete same.