Anyway, switching topics a bit, but bouncing off the theme of what makes something NTR, I had a shower thought the other day. I am 99.999% positive the answer to this is 'no', but: has there ever been a case of 'gender-flipped' NTR? To be clear, I'm not talking about NTS or anything like that, I'm well aware of that genre. I'm talking a story that hits all of the standard beats of NTR, but the usual genders reversed. Like, a female lead who is romantically involved with a male lead, but either in a totally non-sexual relationship or else one that's incredibly tame; then another woman enters the scene who is way more sexually proactive than the FMC, goes "damn this guy is hot, this girl doesn't deserve him", and proceeds to slowly get him hooked on sex with her, etc., etc.
As I said, I'd honestly be kind of shocked if this existed. I feel like the kink of NTR leans way too strongly into gender-specific stereotypes, e.g. the idea of male as inherently sexually dominant and powerful (and thus a man that is not like this being in some way inherently 'lacking'), or the idea of women as being inherently pleasure-addicted, but generally being in denial about it or otherwise unaware until taught. It does make for a bit of a fun thought exercise to imagine what something like that would be like, though!