That's not actually true. In HC2 there's a handful of mom where Aywin and Celis fuck in Noah's general vicinity. There's the lake episode where you wander off after Celis and the pink haired guy and when you come back to the beach Aywin is fucking Zeno in broad daylight making no attempt to hide it or stop when Noah sits down nearby. There's also the implication that while Noah has an emotional and mental breakdown they just give up hiding it and fuck the days away instead of caring for Noah. They don't really ever just start fucking in front of him by choice, but he's inattentive and naive enough to miss multiple situations where they're obviously fucking around him and later on there's at least one scene where he's tied to a chair in front of a one way mirror and is forced to watch his mom and sister get fucked and basically emotionally and physically betray him and the memory of his father.
I am most look forward to how and when the women start screwing around in CotV. I'm hoping Inatari doesn't just keep them to singular partners like N_Taii tends to.