Some sex scenes are videos, instead of a real time render (if I'm not mistaken) it has this option for potato phones like mine.
But if you're worried, it won't make a difference on your experience.
i know that for a fact because of not only this game but pale carnations has pre-rendered scenes too
If your device can run all scenes with the full version then keep it, but if you're having performance problems, maybe crashing exactly on sex scenes, that is when the compressed version comes