So, one thing that a lot of people in this thread seem to not realize is that NTR, Cuckolding, and Adultery are all different, and that each one has a massive amount of variants to each segment.
Allow me to explain.
NTR is girlfriend stealing.
Adultery is cheating at the very base, i.e. simply being in a relationship and having sex with someone who isn't the spouse.
Cuckolding is having sex in front of someone who doesn't get to participate. It tends to have a link with Adultery/NTR, but doesn't necessarily require a relationship.
NTR has multiple sub categories such as NTR A, B, C, Yuri, and Avoidable/Unavoidable.
I forget which letter goes with which but allow me to go in depth about the different categories. Keep in mind that characters and genders can be swapped around accordingly, and that there are variants to these, but these are the main aspects.
>Rape that involves a spouse being stolen.
>The boyfriend intentionally sets up the NTR scenario.
>The scenario happens unknown to the boyfriend.
>Girl on girl in any of the previous aspects.
>Situations that could have been avoided by a character or situations that are out of the characters control.
Cuckolding is a very niche aspect, due to it being based around a person watching.
This is mixed with most of the previous aspects of NTR, but also tends to focus on mixing in aspects of humiliation, dominance, and loss of control.
Most of the differentiation depends on the characters in question, as the personalities of each character can vastly change the tone of a cuckolding scenario. I'll go into some examples of the roles and possible personalities that change the play. Keep in mind that these are not all aspects, but just examples that change these.
Cuckold
>A feminine character who is willing to submit to the bull
>A character who has no intention of losing their girlfriend, but lacks the ability to do anything about it.
>A character who acknowledges their inadequacy and simply allows their girlfriend to partake in sex with others.
>A clumsy/ditzy character who is unaware of the advances being taken on his girlfriend.
Girlfriend
>Innocent and pure, but easily tricked into being seduced.
>Dominant and cruel, having sex simply to spite and taunt the cuckold.
>Slutty to the point of simply having sex to have sex.
>Adventurous and fun, perhaps seeing a picture on the cuck's computer to progress into this.
Bulls
>Big, douchy, alpha-male brutes with just sex on the mind
>A guy who simply wants to get revenge on the cuckold by fucking the girlfriend
>Superior male in every aspect who abuses his superiority
>A bully who finds fun in the situation
>A guy who is simply better at sex and ends up winning priority due to his skill.
Adultery, is simply cheating and can be any scenario so long as one or more partners are in a relationship.
Keep in mind, each kind goes beyond what was explained here, but people seem to not grasp the differences of each one.
TL;DR: NTR, Cuckolding, and Adultery are different. NTR is GF-stealing, Cuckolding is fucking in front of someone, and Adultery is just cheating without context.