It's a RPG and the heroine is a strong, independent female knight. If you want to make her slutty, you have to lose and make her fail at being a knight to corrupt her. Simple as that.
RPGs are about experiencing the story, not winning or losing fights. If you want to role-play character as a slut, then your goal is to lose fights. So you win (get desired result - corrupted MC) by losing.
Of course she could be slutty from the start, have a lover and enjoy sex... but that would be completely different story and character.
You are not playing to win, you are role-playing. Learn the difference.
btw. you usually get a insta-lose skill in NG+ in OneOne1 game. Best way to experience their games is to rush the first playthrough, play the story, learn the game and use 2nd playthrough for porn.
I can see your point, sure you can roleplay to lose. But any good RPG goes together with good story. Make a story so the heroine make some choices that have good/bad outcomes regardless if you win or not, for example, something like this..
By the end of the prologue you encounter the Ex-Knight boss that have his henchmen hold your classmate hostage, now you have an option to..
1. You quickly choose to fight the boss, your classmate got raped, you win but feels bad afterward.
2. You take your classmate place, classmate got free, but you got raped instead, mysterious figure come to help you.
3. You chooses to talk with the Ex-Knight to stall time, he reveal some information about the Knight system in his perspective, you sympathize with him a bit, but as he talks, your ninja friend sneak in and help free the hostages, the two of you fight the boss and win without anyone getting rape.
4. You lose the fight, everyone got rape lol.
So now you can choose the path your character would goes through the story in the Prologue.
with option number 3 being the only "Pure" path, option number 1 would make you distrustful in the eyes of fellow knights and little by little everyone would distant themselves from you, then later you will be recruit by the resistant, a "Neutral" path so to speak. option number 2 would make your friends sympathize with you more, but you will never be the same, you started to like sex and this gradually leads you to the "Slut" path.
So just an easy example here but in my opinion would make the story have much more dept and more interesting, also you don't need to "lose on purpose" this way (if losing is something that is encouraged, making the game harder would help too)