Once again, not exactly sure where people are getting main girl from her.
Also, this is a story driven game, not a sex driven game. Very personality, personal circumstance driven, and if you don't like that then there is always avoiding the game in the first place. If you want a shallow sex simulator, look elsewhere. If you want unquestioned devotion and mindless fucktoys, look elsewhere. If you want a story with substance, this is one of the few h games you'll get that in.
Nothing official or exclusive was decided between the mc and her. The choice to not fuck her, also there. You end up fucking her despite knowing she's a prostitute, that's on you. You can't then blame her or the dev. Those were your choices. Not only that, none of the dialogue screams "love interest" for her. It's just you trying to thank her for saving your sister by trying to become her friend. If you somehow got attached, that's on you. Her being a prostitute was known from the start.
I am not going to say much more but there are a few things I'ma end this with.
Touch some grass. Learn how to have female friends. Stop objectifying women(ironic considering the circumstances, but it still stands since you're only mad she's a prostitute because of the fact she's not exclusive to you).
Not everything is about sex. Not every interaction you have with a woman is about sex. If you see the events of her story as netorase, you REALLY need to learn the definition of netorase cus this don't fit it.