/jp/ - Otaku Culture

None but the protagonist can properly interact with Saya: those who try simply go insane and lose all will, and the only man save Fuminori who did manage to establish a relationship with the girl urged her on to find "her love" — the man without whom Saya would only be a stray fantasy — thus playing the role of her father more closely than most real fathers manage, or strive to manage, to. Keeping this in mind, the scene with the rape was absolutely necessary: it had to be shown that Saya is not simply only for Fuminori to interact with, but that if anyone else invades his fantasy, they will surely ruin it; the world that he created for himself is his world and his only.
But this isn't enough, because as none can enter Fuminori's fantasy, so cannot Fuminori venture beyond it, and he does, does crave to, and the answer given by Urobuchi is mirthfully delightful: the fantasy simply has to spread. Even before the 'seeding' end, there is Yoh. Fuminori could not accept her when she was normal, and even before he rejected the world he could feel nothing about her, only figuring to accept her out of courtesy. But as she became part of his fantasy, utterly cut off from the world and even her own self, a thoughtless beauty with no wishes of her own but to please him, utterly bound, literally so, to be a part of Fuminori's indulgent need, he came to not simply accept her, but to love her, and still then as a piece of his fantasy.