In Roundscape most people are polyamorous, so your companions will/can have sex with each other and npcs. But it's never an NTR situation where the scene is designed to make the player intentionally jealous. It's more built into the story with polyamory. You can interrupt some scenes, by dialogue choice, or just outright ignore them too. Usually those sex scenes will involve the companions inviting the player to have sex with them too. You're the only one that can make your companions pregnant if you're the Male MC as well. The player also never chooses a wife/partner until the end/epilogue of the game. So the marriage options in this game are never officially yours until the end.
As for actual cucking, the player can have sex with two females, a lizard woman(pregnant) and a orc.(If you're a Male MC) Who are married and have a lacking sex life. There is one scene with the Mistress/femdom path of the companion Xyless. Where she will have sex with a demon, and has you watch for her/your pleasure. Which is the "closest" you'll get to cuckhold content against the Male and Female MC. You'd have to read the dialogue for yourself but it's not really cuckhold, more femdom/exhibitionism/slave training. However you can always enslave Xyless and make the "Spirit of Domination" your bitch instead which is more fun.
edit:You can always check out the gallery mod first. And see if you like the scenes/artwork/writing/fetishes before playing the full game.
Talk to the old man near the plants with Meredith in your party. Then leave the zone, I don't remember if you have to swap out Meredith in your party for this part, then renter the zone, and go over to the plants. Interacting with Meredith over at the plants should get you a h-scene of her and the plants.