Ahhhh, so that's why Najaek's route was considered the 'secret route', because he never swung that way to begin with. If the requirement to 'capture' him was to get well with his younger sister, the villainess, first, his reasons to marry the heroine later may be something like, "Though I don't like her that way, she gets along with my family well, especially my sister who I love dearly. If it's her, I can put up with a commitment like marriage."
Though I'm not really into the borderline feminist ideology of 'marriage between man and woman won't lead to happiness', it really puts into perspective how things were intended to play out in the otome game. Like, what were the devs even trying accomplish with such a plot?