I think the whole relationship thing should be up to the player if they want singular or group dynamic. If we go by how the Spirits may act then the closest we got in Myths and Fairy-tales then Fairies themselves are willing to share under certain conditions.
Another thought is instead of worrying about a full player model how about do what another hentai game did so far. Just concentrate on the sexual poses of the characters and where the player would like to place their hands at the said time or the option of what to do with their hands. As will as special positions based on where the location the player is doing it and if items are required as will.