Correct me if im wrong, you are asking whether the mc is even needed for ntr? or you dont need ntr to make a slutty main heroine?
to answer:
People like the comparison between the girls when they are with the mc vs with the ntr partner. A good example of this is there's a game made by
GAPTAX named
Battlearmor Sorgante, long story short its a mecha game where the mc goes to mission and is framed and is blackmailing the main heroine.
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here she is being harassed while working (above mc perspective)
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And here is another much like in the game, a heroine stats tracker
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and like here she also falls
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To summarize it is not wasted kurt being oblivious, as the player you are seeing the build up makes it more compelling as a story or character if the female is not
just a slut. people wanna see corruption and hidden sex that makes it more thrilling and enjoyable.
sidenote:
I just wanna share that gaptax also has another game named raiohgar, also same premise of corruption and ntr behind the mc's back
there is also a game in development by Nukomajin with same ntr corruption hypnosis
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I'm waiting for this game while waiting for bakunyuu party to finish. So I'm waiting twice as fast than you guys
D