I see.
The girls are tied to their content
Yes and no. If it's about sharing you might want to read the FAQ in OP, especially the last line of this paragraph:
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Is sharing always optional? Will it close a character's route if I decline?
Sharing is always optional. There are a few characters that will not date you however if you don't want to share them. Suzy and Tammy for instance, you will have to share with each other. Izzy is another that tells you very straightforwardly that you will not be the only one she's having sex with.
These characters, along with one or two more - if you decide that you do not want to share them, their stories will end.
Most of the rest of the characters will have sharing as an option for one or two scenes, but these scenes are entirely optional.
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You will get the option to opt-out of sharing on a scene-by-scene base.