The logic there is quite simple: the obelisk allows you to look again at those scenes that you have already completed and they are available according to the script only 1 time. Among them there are quite a lot of 18+ scenes. Essentially, these are Celica's memories.
Therefore, if you want to watch how the slime brothers give Celica a professional massage, you can do this simply by visiting the forest in the evening or at night. These animations are available in normal gameplay and therefore there is no point in adding them to the obelisk.