It's mostry text-based, there are more or less 8 CG, so it doesn't have CGs to go along with every sex scene, but usually is present a naked spite of the NPC. It resembles ToS very much, in fact there is a crossover event between the two in both games.
Depend what do you mean by monogamous relationship: if you talk about sex scenes with even enemies, you can choose to attack instead of flirting, if you talk about NPC's hearts you are "forced" to advance your relationship with two to progress the story, if you tali about having sex with NPCs, you are only "forced" to have sex with Bernard the shopkeeper. So if you want to have sex with only one person you must only attack the enemies and persue only Bernard