As a life-long Pornography enthusiast, I got to give my 2 cents when I saw people talking about porn fetish and writing.
For some, the first NTR epiphany was probably when the first girlfriend first cheated on you, and you confused the increased heart rate as hatred, but only later find out it was excitement.
Or it simply started when you watched a main stream movie such as Indecent Proposal (Demi Moore at her prime!), and your body reacted weirdly, you kept telling yourself you hated the plot, but your dick just hard as rock the whole time.
Psychological trigger is the highest form of any fetish. I've studied countless of games, novels, manga, video, films, articles, and forums, across multiple social cultures and norms. The works of the Japanese are the most creative, the most edgy, and the most 'disgusting' (in a good way) amongst all the NTR materials in the World. Their best works always go for the mindfuck.
Four particular manga come to mind for research or dedicated studies, all have great setup to create situations and characters that fuck your mind up. So fucked up and yet so hot that could possibly turn NTR haters to NTR lovers. Warning! some NTR lovers said they needed a break after reading any one of them. Consume at your own risk.
Fork in The Road by Mashiraga Aki
Musa by Mashiraga Aki
Karami Zakari by Katsura Airi
Netoraserare by Shikishiro Konomi
p.s. porn is an art, and should be appreciated as such.