This is the ridiculous truth of it. It was abundantly clear that letting Kathryn go to the dance with Pat alone would be an opening for the start of the NTR path, and by going with her you'd cut that off, or possibly start an NTS path with Pat (although that would take some mental gymnastics from Luke or humiliation from Kathryn to pull off). Yet when it actually came to it, the best option is the exact reverse, because of the convoluted argument and outcome of the argument. It's really poor decision making from the dev.
The easiest/best thing would have been to have a Kathryn choice, during the dance evening, of whether to go home with Luke or stay with Pat. All it would take would be a few additional renders of the couple going home together, you wouldn't even need any/much dialogue as they could just be ignoring each other after the argument. You'd still have the stupid argument on that path, but it would negate the even bigger stupidness of forcing Kathryn choosing to stay with Pat. If you're talking about the drama being required for whatever reason, the argument itself can bring that. That way, you would (mostly) please both parties while still having the doubts and drama available if it does turn out that it's "absolutely vital to the ongoing plot".