- We cannot permissibly intend to produce a person for reasons that do not include the specific person's own good.
- We cannot intend to produce a person for reasons that include the specific person's own good.
- We cannot permissibly intend to produce a person without reasons.
- Necessarily, if we intend to produce a person, we do so for no reason, or for reasons that include the person's own good, or for reasons that do not include the person's own good.
- We cannot permissibly intend to produce a person. (1-4)
Now on its face, premise (2) is false. Surely people do procreate for the child's own good. But I don't think so. They may be acting for the good of whatever child results from the reproduction, but there is no specific child for whose good they are acting. And when they act for the good of whatever child results, the specific child's good ends up being a constitutive means to the good they are seeking, so they do not escape from the Kantian criticism. The good of a person is an incommunicable good: it is that specific person's good. But the existence and identity of the child depends on the couple's decision in a way that the couple is unable to figure out beforehand. Thus the couple cannot be deciding in light of the identity of the child, and hence cannot be acting for the good of that specific person.
Note that God does not suffer from the cognitive limitations that give rise to (2): he can know our identity before he decides to create us, and can decide to create us for our own good.
Now, when a couple engages in the marital act, they have a reason to engage in that act apart from reproduction: the act is good in itself, being an embodiment of marital union. Thus they can act so as to unite, and accept the child as a gift from God that goes beyond their intention. Note that even given my argument they can permissibly rationally consider the reproduction in their decision whether to make love, for instance as a defeater to various defeaters (being tired, etc.) to the marital reasons for lovemaking. On the other hand, in non-coital methods of reproduction like IVF the couple is specifically intending reproduction, and that is wrong if the argument succeeds.
I am not myself entirely convinced of (1), because I am not entirely convinced of the Kantian autonomy framework. We aren't ends in ourselves: we exist as constitutive glorifications of God. Thus it does not seem contrary to the dignity of a person to be produced for the greater glory of God. Kantianism is what you get when you remove God from the story. If that's right, then we get the surprising result that only theists can permissibly intend to produce a child. Atheists, to be consistent, will need to have the Kantian attitude, and while they can permissibly reproduce, they cannot do so with the intention of reproducing, if everything else in the argument works.