Unfortunatly you have to understand this is almost impossible to do that without hurting the story and incoming event.
This game is story driven, some event will be mandatory, some other will be optional and will have choice that may lead to something different.
Guys i want to be clear, im not inventing anything here, if you want a lot of choice, customisation, free map movement etc, you will get a 'grind' game with a : "do this 10 times to unlock this" ,those kind of game can be good but it's not the kind of game I'm doing.
A story driven game is an option, although it looks more like a video or a book than a game, since the player has no options in the end. Either he follows the story written course or it's game over.
But a game which allows the player to choose the character he wants to have sex with and what kind of sex to have with her, doesn't automatically mean a 'grind' game:
It can be a 'grind' game if devs insert 'intelligence' points or 'strength' points or whatever variables and a given value to them in order to progress with some character.
But it can also be a game with many choices, without being a 'grind' game, when your progress with each character depends on how you interacted with each of them, if you interacted in a positive way with a character, you progress. If you interacted nagatively with a character, your relationship with that character won't progress.
Having options to try to progress with the characters you are more attracted to, and not being forced to have a romance with a character you despise, is more fun for the players, in my point of view.