
Hit the nail on the head there. Your primitive brain can’t tell the difference between a real interaction and a simulated one. Hell, the simulated one is designed to be more addictive as it conforms to you whereas another person would have their own wants/needs/desires that could conflict with yours.
And while you do have a point, there’s usually at least a small percentage of real desire in a fantasy. I just don’t think most people could suspend disbelief for long enough for NTR to be “sexy” to them.
We also can’t discount that IRL traumas can factor into a person’s perceptions of kinks. Someone who’s loved through infidelity, pretty common these days unfortunately, is less likely to find porn glorifying it to be enticing. Most likely it’ll trigger revulsion.
But, as with all things involving humans, there are no absolutes. People are weird.