Jeremy's argument is that it is a good to seek natural children, but it is also a good to not be selfish in doing so. Both are obligations. Hence, since one has an obligation to not be selfish, and this same person who is selfish seeks a child for selfish reasons, it is better for this person to not fulfill the good of seeking the child so that one will not be in violation of the second obligation.
In essence, Jeremy's argument is that if a good is done for the wrong reasons, and it is also a good that should be done for the right reasons, then there is a moral obligation not to do the first good if it would bring about a violation of the second.
I left a comment that has not yet been posted, so I'll just post it here:
Let's apply this to other things too. If you have a selfish attitude in giving to your fellow brother in Christ, who is poor, it's better not to fulfill the obligation to give to him, as you could be doing more harm by making yourself think that you're a really good person. If you don't give, you don't have more fodder toward that attitude and may, therefore, have a greater chance to repent of the attitude. If you have the same selfish attitude in feeding your children, it's better not to fulfill the first obligation to feed them. I'm sure the desire to not disobey a supposed second obligation will be a great excuse for not obeying the first on the day of judgment. I'm not quite sure if God's going to buy it though.
Of course, following this line of reasoning with natural children assumes that God isn't the one giving a selfish person children in the first place, unless you want to say that God is doing something immoral by giving a child to a person with the wrong attitude, and thus, giving the child over to further damage. If it is not immoral to place a child with a selfish parent, who may "damage" a child, why is it immoral to have a child as a selfish parent? It is selfishness that is immoral, and to seek to build the web of sin rather than repent in the first place is not a Christian response to sin. Is it honestly better to prevent the existence of that child than to have him grow up with a selfish parent or two? And since we all have sin, and are incredibly selfish, even mixing selfishness with right thinking, I take it that no one should ever have children, so that rather than learning to love in fulfilling the first obligation, even while we are selfish sinners, we continue to be selfish and worry more about our grandiose affects upon a child than the giving of life to that child in the first place. It seems to me that this argument is simply a way to stack selfishness and the justification thereof on top of one another.
First, we are told in Scripture that we are to do good to all men, specifically to the household of God. We are told that the doing of good is how we judge whether a widow is put on the list to be cared for by the church. We are told that Christ will look at the good deeds we've done on the day of judgment. We are NEVER told that if we have a selfish attitude toward doing good, we shouldn't bother doing it. Motives are not the issue in the doing of good. In fact, there are selfish motives in not wanting to be judged and assigned a place with the wicked who don't do good. There are selfish motives in wanting us to feel good about doing good. There are always selfish motives within us because we are always sinners simultaneously justified. Hence, we ought to always do good and seek to correct our motives in those works, not in the denial of them.
Second, the argument that one is doing a further non-good by doing good assumes that an evil is not being committed already in a disposition of selfishness. In other words, the reason why one has selfish motives is because one is selfish. He is not less selfish because he refrains from doing a good for selfish reasons. He may, in fact, be more selfish because he is so consumed with himself that rather than care for a child, the poor, a widow, etc. and think of himself as a selfish person for doing so, he would disallow the child's existence, starve the poor, and let the widow be thrown out of her home. I can think of nothing more selfish than that.
Third, based upon the previous two observations, what needs to happen is for the selfish person to correct his attitude in the doing of good, not refrain from doing good in order to check it. That is completely counter to the argument in Scripture. It says with the unfaithful servant, "I knew you were an exacting man" (Luke 19:20), so upon that basis, seeks only to preserve himself in refraining from doing good, so that he might not be judged for it.
Fourth, selfishness isn't primarily an attitude in Scripture. It's an action. It would be nice to have it as both, but if one has to go, it's not the action, which Scripture sees as primary. Notice that even in the preaching of the gospel, Paul doesn't argue that those who have wrong motives should stop doing it, but rather that he is pleased about the work of good, the preaching of the gospel being accomplished, even through men with false motives. If he were to correct them, then, as he is likely doing here, he would correct their false motives, not their actions.
Fifth, it assumes a vehicle toward breaking our selfishness is in the refraining from good rather than in the obedience to do good (i.e., the seeking to do good even in the face of our own selfishness). In other words, what is the cure for selfishness but a life of selfless acts? In the doing of good, one learns to love others. I don't see how Jeremy's argument doesn't suggest that one should hide away in a closet somewhere, and never interact with others if their interactions are for selfish reasons; and yet, it is in the very interaction with others that one learns to become selfless. I see no difference here in the seeking of children, as one may begin one's journey of raising children in selfishness, but children are often God's instruments that teach even the most hardened of sinners to love, especially if one is a Christian, since God works all things to conform us to the image of His Son, and He would certainly use children as a means to do so, and use us as the means to do the same for them.
So Jeremy argues some good and brilliant stuff most of the time, but not so much this time.