Exist 3 kinds of NTR :

NTR kind A ( consensual sex )
the girl that the protagonist love agree to make sex with other guy instead of the protagonist in the game, she is not forced, she want it, is NOT a rape .

NTR kind B ( semi-consensual sex )
the girl in the beginning of the game is really raped, but, during the gameplay this rape begin to change gradually to consensual sex, because the girl begin to like it a lot

in this kind of game ( kind "B" ) , are listed games that are used some kind of "trick" to make the girl to make sex with others guys instead of the protagonist, like hypnotism, aphrodisiac or sometimes even brainwash . All games with NTR related with this kind of "feature" are considered "kind B".

NTR kind C ( violent rape without pleasure )
the girl is raped - she resist until to the last minute, she never agree with this and she never feel pleasure during this sexual act , she feel only pain and suffer, this is just a violent RAPE really .

For me , fhc from Brazil, NTR only exist when this happens with a wife, bride or a girlfriend of the protagonist ( a girl that have a official love relationship / commitment with the protagonist of the game ) and the protagonist really love this woman very much, and this woman love the protagonist very much too, however, for some reason, she is forced to make sex with other man ( maybe bacause of a “uncontrollable sexual desire”, or maybe because of blackmail, or like the “payment” of a “high debt”, or, just a normal RAPE ) I not know, the reason will be explained in the game, but, the fact is that the girl need to love the protagonist and the protagonist need to love the girl. NTR without love, for me, is not NTR.