Mostly just whiner's NTR, primarily centering around the fact that Cassandra has sex with her boyfriend Mel, but you can also fuck her (so technically, you're doing the cucking); basically, characters aren't exclusive to the player character. There's also a few voyeurism antics in the game with invading people's minds and living out a day as a spectator to their lives (including when they have sex).
In terms of "actual" NTR, you can make your succubus fuck a target character, and you can also mind control people to this same end as well. And it's not like you're having your love interest fucked either; the player character isn't in a committed relationship for this to be the case, nor is there any serious romantic interest thus far (the only exception being one character, Julia, that's established as the player character's usual squeeze).