Dunno where people are getting the idea that the daughter isn't a blood relative. There's maybe a vanishingly small percentage chance she's not, and a transitively massive percentage chance she is.
The game doesn't go out of its way to confirm at any point that Amanda is unrelated. If anything, it instead goes out of its way to make the MC feel more and more guilty about the relationship because the more he thinks about it, the fewer doubts he has -- and none of the other characters in the story have any doubt that she's his daughter. The dad couldn't control himself and came inside Lanie, and then nine months later, the daughter appears (and, incidentally, he later does the same thing to the daughter by mistake, and he gets pretty worried about it). Just because Lanie never explicitly said that the MC was the father is a pretty shitty justification to believe that he isn't, especially when he was convinced that she was, and took immediate custody of the baby.
The story is, in other words, based on implicit writing, rather than stating everything explicitly. For instance, even the part where you get to name her gives you the option of whether it was the name that Lanie always wanted or the name that the MC always wanted.
It seems that some players take the implicit parentage rather than explicit parentage to mean automatically "not my kink then", which is plainly silly. Really, the fact that it's not explicitly stated is merely meant to give people an "out" in their own moral justification, while still all-but-confirming they're related for those who don't care or for those with the kink.
Actually, I find that part of the brilliance of the story; although the game did indeed get quite mediocre, especially where it dragged through the last few chapters (seriously, I felt like smacking the MC across the face every time he bitched and moaned during his court case), and they milked a few sparse releases and simultaneous-projects, it still plays quite a bit with the player's own morality in determining just how far they're willing to go and whether they're gut-punching the girl by having an open relationship with anyone else.
(If we're talking about "not my kinks", my real complaint is that the player is more or less forced into anal sex with her on an optimal playthrough. Not everybody likes the chocolate tunnel of love, especially when it's definitely not chocolate in there.)