To put it simply, They just attack what they can't let themselves understand, find the time to understand or find the truth of. They feel a sense of danger from pursuing the truth and maybe they are in danger, who knows what kind of environment they are in or grew up in? That's why they're so heated about it. Maybe enjoying NTR games will totally flip their minds upside down and they won't know how to function normally after, maybe they won't be able to get it up when having sex, maybe they won't be able to see their crush in a normal way anymore etc.
All of these are not gonna happen of course but the fear is just too strong.
Real Life NTR on the other hand, that's a whole can of worms I'll leave that to actual swingers and cuckolds. I don't know its long term effects, I don't wanna try it, I don't want to entertain it, it frightens me.