Let me write about my own classification of NTR, which is a bit more detail than general classification. IMO NTR is basically a cheating/affair/adultery story, based on the fact that the heroine(s) have intercourse with other male(s) other than BF or husband. There's no clear boundary among different branches of NTR so it is not easy to categorize them.
1. Basic R/L/S classification
a) Netorare (NTR) = Story described from MC (main character = husband or boyfriend)'s point of view. Protagonist takes the heroine from MC. Usually MC's reaction differs widely based on background and personality, or changing through the story.
b) Netoli (NTL) = Story described from protagonist's point of view, seducing and taking the heroine of MC. Sometimes NTL story also describes the heroine's story to depict the corruption or change of heroine's attitude towards the protagonist, which is also important to distinguish NTL genre from usual just-rape / just-humiliation genre.
c) Netorase (NTS) = Story described from MC or heroine, but NTS is more or less close to swapping or sharing. MC let his heroine cheat and have intercourse with protagonist, and heroine reluctantly takes it or willingly takes it.
2. Classification based on MC's reaction
a) MC have no idea about cheating/seduction of his heroine, and he never catch a single clue until the end of the story. Some NTL story takes this route, and concentrate to show the progress of seduction and fall of the heroine. Sometimes, the story ends with the cuckoo-pregnancy of heroine, implying the father is the protagonist and MC have no idea that the baby is not his. (My personal favorite genre)
b) MC finds out (or knew from the beginning) about the affair of his heroine, then he resists. He tries to control the situation and turn everything back to normal again. Multi-ending is possible based on heroine's behavior to MC's reaction.
c) MC finds out (or knew from the beginning) about the affair of his heroine, then he watches. He does nothing, no resist neither encourage, just watching his heroine seduced by protagonist. Sometimes MC hopes his heroine to return to him again, and try to be good with heroine instead of mad at her. But in some stories MC really gives up hope and watch in despair. Some cuckold story can be categorized in here.
d) MC finds out (or knew from the beginning) about the affair of his heroine, then he encourages. NTS / cuckold story can be categorized in here. In some stories, MC initiates the situation by let his heroine sex with others and persuade his heroine, and she voluntarily or reluctantly follow.
3. Classification based on heroine - protagonist relationship
a) Cheating / Adultery, consent = She accepts the protagonist from the beginning or early stage.
a1) Heroine and protagonist are loving each other
a2) Heroine is conservative and consider MC seriously, but protagonist is excellent womanizer / Casanova.
a3) Heroine is slut, but protagonist is very good and nice guy, and he is serious about heroine.
a4) Heroine is slut and protagonist is womanizer.
a5) Heroine and protagonist started "firework" due to curiosity or for fun, but the firework become strong and both get serious to each other.
b) Non-consent/reluctance = she resists and fights, but eventually/reluctantly accepts at later stage due to pleasure from sex (plus other reasons / motivations) Relationship become consent later. Protagonist is charismatic or persuasive, or at least he is really good at sex.
b1) Relation starts with blackmail, and heroine is seduced after continuous forced intercourse. Usually, heroine is corrupted due to pleasure from sex with protagonist, but some good stories also introduce good motivation / reason why and how she falls.
b2) Relation starts with corruption, and heroine is corrupted by alcohol, drug, hypnosis, or mind-control, and heroine is seduced after continuous forced intercourse.
b3) Protagonist is famous general, businessman, richman, king, royal family, or politician. He use his assets, money, physical or political power to seduce heroine and to have relation with her.
c) Rape = she resists and fights, (or have no idea) and she never accepts.
c1) Heroine is forced to have relationship with protagonists and hates it all the time. Never surrender. And she never feels pleasure or ecstasy from continuous forced intercourse.
c2) Heroine is forced to have relationship with protagonists and hates it all the time. Never surrender. But she feels the pleasure and ecstasy from continuous forced intercourse.
c3) Heroine is forced to have relationship with protagonists and hates it. But her mind is eventually broken.
c4) Heroine is forced to have relationship with protagonists. But she never knows the fact that she is forced to have an intercourse with protagonist, because she was drunken, drugged, or sleeping.
d) Group - can be a mix of a), b), c)
4. Classification based on intention of protagonist.
a) Badguy = Protagonist approaches to heroine, just to destroy the relationship between MC and heroine. Usually because of revenge, or protagonist is crazy womanizer or etc...
b) Romance = Protagonist crushed to heroine, and try to take heroine from her MC. Love-triangle situation, but heroine's behavior and situation around them decides the genre: NTR or just another teen novel.
c) Crazy world = everyone is living in crazy fantasy world, where NTR or cheating or polygamy is allowed and perfectly normal.
d) Induced = Protagonist is also tricked to have a relation with heroine, or someone push protagonist to take heroine from MC
d1) NTS = It is MC who push protagonist to have relation with his heroine.
d2) Swapping = 2 couple agree to have NTS and change each's partner.
5. Classification based on heroine - MC relationship
a) Lover = Heroine really loves MC and MC really loves heroine
b) Kinda lover = heroine and MC have a good intention and feeling to each other.
c) Unrequited love of one side = MC loves heroine but heroine never knows MC loves her. (Or opposite way)
d) Family = sister or mother
6. Motivation of heroine (slightly overlap with number 3)
a) Heroine has some degree of nymphomaniac. She just loves sex and easily seduced by orgasm and pleasure of sex or big dick or creampie.
b) Heroine is not slut but unsatisfied with the relationship with significant one. Husband or BF is too busy or travels too far. Or they are emotionally careless, indifferent, boring, nerd, regard heroine as trophy wife. Or they are just too weak, no stamina or too small penis or premature or etc. Whatever reason, husband has some problem(s) which induces the heroine another good motivation.
c) Heroine is too naive or too stupid, so that she has no idea about trap of protagonist, or unable to distinguish the temptation and friendship or good will. Or, she is too sympathetic to other males.
d) Heroine had tough childhood (too strict father, divorced parents or
domestic violence or etc...) which causes her to suffer from lack of affection and attention. This makes her more or less easily accepts the temptation from outsider, or at least make her moral standard grey / vague under specific circumstance.
e) Heroine is conservative and has normal standard, but one night mistake changes everything...
etc...
7. Based on protagonist's job / position in society / personality.
There can be too many possibilities. It can be friends, boss, teacher, mafia or gang, plumber, repair man, dirty old man, driver, etc....
8. Based on how MC finds out the NTR / cheating situation
a) MC never figures out what is going on until the end of the day = Story ends with affair between heroine and protagonist. Maybe epilogue implies that MC finds his kid not looks like him, and they looks like his neighbor....
b) Someone let MC know about the story = By anonymous pics or video, he finds out by friend's message or phone call with some evidences.
c) MC accidentally finds out the secret = He witness the moment by chance, in classroom, in their home, or in any other place.
d) MC finds out the change of heroine = Heroine pierced or tatooed and MC finds it. Or heroine refuse the relationship with MC.
e) Confession from protagonist or heroine.
etc...
9. Based on ending.
Bad ending / sad ending / happy ending / 'Crime watch daily with Chris Hansen' ending or etc...
