Ah cool, as I am a moral anti-realist and you are an epistemological moral skeptic, we're both interested in thinking carefully about what kinds of moral arguments are convincing. Since we're talking about terminal moral values at this point, the "arguments" I would employ would be of the form "this value is consistent with these other values, and leads to these sort of desirable outcomes, so it should be easy to imagine a human holding these values, even if you don't hold them."

I [...] didn't hear you actually advance any positive justifications for a near-zero moral value on genetic closeness. If you'd like to do so now, I'd be glad to hear them.

Well, I don't expect anyone to have positive justifications for not valuing something, but there is this:

Consider a more humanistic conception of personal identity: Your child is an individual [...] who has a special personal relationship with you.

Consider a more transhumanistic conception of personal identity: Your child is a mind [...]

So a nice interpretation of our feelings of filial love is that the parent-child relationship is a good thing and it's ideally about the parent and child, viewed as individuals and as minds. As individuals and minds, they are capable of forging a relationship, and the history of this relationship serves as a basis for continuing the relationship. [That was a consistency argument.]

Furthermore, unconditional love is stronger than conditional love. It is good to have a parent that you know will love you "no matter what happens". In reality, your parent will likely love you less if you turn into a homicidal jerk; but that is kinda easy to accept, because you would have to change drastically as an individual in order to become a homicidal jerk. But if you get an unsettling revelation about the circumstances of your conception, I believe that your personal identity will remain unchanged enough that you really wouldn't want to lose your parent's love in that case. [Here I'm arguing that my values have something to do with the way humans actually feel.]

So even if you're sure that your child is your biological child, your relationship with your child is made more secure if it's understood that the relationship is immune to a hypothetical paternity revelation. (You never need suffer from lingering doubts such as "Is the child really mine?" or "Is the parent really mine?", because you already know that the answer is Yes.) [That was an outcomes argument.]