The last time I played (like year ago or so) there was no "vanilla"/monogamy route: if you ask your mom to be present during her sex with your girlfriend – this opens the NTS route, which means the girlfriend does
not hide from you sex with monster girls/MILFs and so on (and tells you what is happened). Otherwise, the girl will lie about what happens during her time with the characters without you (but the MC will still be peeping on her, so he know about it). But I only played NTR route a little bit because I like NTS more, so I don't know how NTR will work further.
UPD: OK, now I understand what you mean:
And in the gallery only protagonist-oriented scenes and NTS-oriented scenes.
The developer probably decided to abandon NTR route.