I love you, thats why im going to pledge my life to protect the other girl and have sex with her while you watch but dont worry i still love you
UUUUGGHHHHHHH
I suppose it could have been the other way around, where she took on the responsibility of having sex with Fusuo and does it solely out of obligation, and genuinely fell in love with Shigure at some point. Read that way, she may not be able to get out of the other obligation and is stuck in that pattern, which would be a very Japanese sort of story in the end.
Doesn't necessarily make it better from a NTR point of view, but it does make for a more interesting story.