Because she is so excited about being your girlfriend, anticipating, hoping she does the right thing.
Sorry, even to a cartoon, being a D to some one, is just a bummer. No one wants that. If she was a BAD person, or at least we did not even SEE her thoughts, or know what she felt... then that would be fine... but we are not in that situation.
We're in the situation that this girl is in love with with us, wants more, and we have to AVOID encouraging her, giving her the wrong idea....from a position of having already, to a degree, done so. THAT is NOT COOL. Just real bad vibes yo.
If you do not understand that, then I guess you are a sociopath.
Who wants to have to deal with seeing a woman's rejection, being privy to her internal narrative when it happens? NO ONE sane.
That is why this cheap shot is a fucking mortal sin in the AVN genre, and any game which makes it, is lessened by doing so. There should be NO hurdles, of ANY kind, to the MC pursuing ANYONE in a game like this, at first. If there turns out to be some, they should be of his OWN MAKING, not because the Dev made the worst mistake you can in this kind of game.