There are special factions that can kidnap/capture girls and then train them while they're in captivity. When the character 'falls' during training then they start fighting for that faction, and lose any relationship traits such as Love/True Love when they do so. I admit I don't know if they still lose it if you go the slavery/submission relationship path, but I assume that's not relevant for NTR purposes. For the special factions to show up, you need to start a new game, go pick what faction you play/establish (pick special if you want to start in them), and then a list of factions and numbers should appear, as well as a choice to if they start immediately or a few turns in the game. There are no special lines/dialogue for characters in love aside from the loss of the Love/True Love, though.
Going to the diplomacy screen and outright giving characters to factions is something you can do, though as far as I'm aware only the special factions actually do anything sexual to them afterwards. Note that this is outright giving and not having them serve. If you're referring to just having girls do sexual favours for foreign rulers, that's also an option in diplomacy, but there's nothing indicating that it's NTR aside from loss of favor and submission.
On a side note, if you change who you play as using in-game cheats, then the girls will change their relationship status to whoever you're playing as, so there's no way to be the guy doing the NTR either.
Aside from that, there are NTR daily events. If you want to maximize the odds of getting them you can fiddle with the event settings. I believe the Millionaire one is one that was mentioned somewhere, and he literally kidnaps a girl. The aforementioned exploring is technically NTR but never mentioned as such in-game, treated pretty as raping them as if they are single.