You don't need to be in love to have sex and sex can be and is amazing even being casual, because sex is about attraction, compatibility and technique, that's true. But fucking and making love is not the same, just like a bro hug and a romantic hug is not the same, and having sex with a person you love is just insuperable, even if the sex is not the best.
If you are playing this game just to jack off, Eternum is not your game because the girls and their personalities, the relationship between them and the MC and the story (gems of doom, MC past, what really is Eternum) is as or ever more important than the porn in this game, and that porn is built on the premise of love between the girls and the MC, not just sex (they are some scenes that are just sex in the game, but is not the norm), so my advice is: just play another game, because you are searching in the wrong place.