In the past, one might have argued, "You shouldn't have sex with your daughter because it would be better for her to marry someone closer to her own age, who has a different set of genetics and can therefore produce better kids."
This argument always did have the problem that some girls just happen to prefer older men. But whatever.
The problem now is, the typical girl is not going to get married at 13 like in the old days, but will instead become a slut. She'll get fucked by some Chad who will dump her, and then she'll end up getting fucked by lots of other guys, and have a kid outside of wedlock by one of them. If she ever does get married, the marriage will probably fail.
It isn't just trashy girls who are like this; middle-class girls are the same way. Not only that, but even if she does have a seemingly "successful" marriage (i.e. she and her hubby don't split up), she'll still probably only have like one kid -- not enough to sustain the family line. Plus she'll probably cheat on her husband.
Rather than let all that happen, it's probably better, if you have a daughter, to have a quasi-marital relationship with her, starting when she's at a young age. One may as well be the one to pop her cherry and repeatedly get her pregnant while she's still in her teens. That will be good for her kids, too, because they'll have the benefit of having lots of siblings to play with. Plus they can go hang out with their grandfather whenever they want, because he'll be the same man as their father, who lives with them in the same home.
The only problem is, the mother may get in the way of this plan. What you have to do, then, is maybe wait till the kid reaches a certain age where she's no longer breastfeeding, and then bump off the mom. Maybe tell your kid's mom, "Honey, let's go on a romantic getaway, just the two of us" and take her on a vacation to an exotic Caribbean island, where you will teach her how to scuba dive through a 17th-century shipwreck. Unfortunately, a tragic accident will befall her in which her scuba gear gets tangled up and she becomes trapped in the wreckage. You, of course, were running out of oxygen yourself and had to surface, leaving her there. By the time you returned to try to rescue her, she was already dead.
With the mother (who was probably starting to get kinda old and wrinkly anyway) out of the picture, you can then become the sole parent of your daughter, and molest her at will. It'll be the same scenario as in the novel Lolita. Hopefully, though, you'll get to her when she's young enough that you won't have to deal with some interloper like Quimby.