Suppose a couple uses contraception but conception nonetheless occurs. Then it is true that the couple have put obstacles in the way of their own child's life, indeed that they have engaged in activity directly opposed to their child's life. To stand to one's child in the relation of having directly opposed Johnny's life fits poorly with having an unconditional love for Johnny, unless one has repented of having opposed Johnny's life. But if the act of contraception was morally unproblematic and rational, then one cannot repent of it, since a part of repenting of an act is recognizing that the act was not to be done.
Therefore, contracepting couples take the risk (for surely there is always a chance of pregnancy) of standing in an inappropriate relation to their child—in the relation of having striven against that child's life.
I don't know how much this argument establishes. In the case of 100% effective contraception (e.g., complete removal of the ovaries, fallopian tubes and uterus), the argument is silent. In the case of typical contraception, whose effectiveness is less than 100%, the argument either establishes that the contraception is wrong, or that at least there is a very strong moral presumption against it.
Does the argument say anything about the couple who uses natural family planning (NFP)? Well, there are two ways conception can occur despite NFP. One way is because the couple engage in marital union despite the fact that the NFP method tells them that there is a significant chance of conception. If Johnny is conceived in this way, then the couple has not done anything that hindered or opposed Johnny's conception. (One might think that by abstaining on other days they hindered Johnny's conception. But Johnny could not have been conceived on those other days—they abstained from conceiving other children then.) In fact, in this case, it is right to say that the couple wasn't using NFP on the relevant day.
The other, and apparently rarer, case is where the couple mistakenly believe that they are infertile on a given day, but in fact they are fertile (either because they incorrectly used the NFP method or because the NFP method made a mis-prediction). In this case, I do not think we can say that the couple hindered Johnny's coming into existence. But we can say that they hoped Johnny would not come into existence, and that they would have refrained from bringing Johnny into existence if they knew there was a significant risk. The relation of having hoped one's child would not come into existence, and of its being the case that one would have refrained from bringing the child into existence, is not an ideal one. Thus there is a presumption against risking being in such a position, and hence there is a presumption against using NFP (which is closely related, I suppose, to the Catholic claim that a couple needs to have serious reasons to use NFP). But this is not a relation that is as morally problematic as the relation of having actively tried to hinder one's child's coming into existence. It is one thing not to have striven to further a child's life and another to have striven to hinder it.
But could one perhaps say that the NFP-using couple in the second case was trying to abstain from the act that would have produced Johnny, and trying to abstain from conceiving is an active opposition to life? But it seems to me that if it is an opposition to life at all, it is a much lesser one than active hindrance—just as it is one thing to try to abstain from giving an extraordinarily burdensome medical treatment (this may be hard—it may require a struggle for a conscientious health professional to refrain from offering the treatment) and another to kill. And perhaps the intention in abstaining is not to prevent Johnny from existing, but to prevent oneself from acting against the virtue of prudence (by having potentially fertile marital relations when one has grave reasons to the contrary).