Funny you should ask -- there's a doctoral dissertation now being written on exactly this question (I am marginally involved in its supervision). The student essentially argues for the conclusion you suggest, claiming that, in the world as it is, those who decide to have children at all ought to adopt rather than conceive. Adoption confers a huge benefit on a child who would otherwise grow up under conditions of institutionalization and deprivation (for example, in an ophanage in Cambodia or Niger). And adoption does not take away a benefit from anyone: the person one would have conceived will simply never exist.
There's no stance of the academic community on this issue, yet. Time will tell whether the student's view will be widely accepted or rejected. It's bound to stimulate discussion if only because most affluent people believe that they have every right -- not just legally, but also morally -- to conceive if they wish.
The student's thesis might be opposed on behalf of the people who, if her prescription is followed, will not be born. But this opposition strikes me as unpromising. Perhaps it is good that more people can enjoy life rather than fewer. But one must surely balance this against the negative effects that additional people have on the quality of life of the present and future generations. All things considered, I don't think we have moral reason to inflate the human population from the present 7 billion to 8 or 10. And it seems entirely permissible, then, to refrain from conceiving -- no matter whether you then adopt a child or not.
The student's thesis might also be opposed on behalf of people who really want to have their (biologially) own child. So abstractly stated, this desire may seem a bit self-indulgent. But it may become more understandable with context. For example, a couple may think of their child as a celebration of their love for each other, with each wanting their child physically to resemble -- not so much oneself, but -- the partner one loves. Or think of a very musical family caring deeply about having a child that shares their gifts and devotion (the horror of Mozart junior turning out to be tone-deaf). To be sure, conception does not guarantee that one's child will share the great musical talents of the parents -- but it surely substantially improves the odds over adoption. And ditto, of course, for other heritable traits such as athleticism, beauty, height, mathematical ability, and so on.
Many parents facing the choice may also be worried about the time their potential adopted child would already have spent outside their care: about the nutritional deficits this child may have suffered, with consequently diminished development of brain and body; about traumatic experiences the child may have encountered, possibly resulting in excessive distrust and other anti-social tendencies; and so on. These worries could be addressed, at least in large part, by improving the current child care and adoption systems. But in the world as it is, they are real worries that lend moral weight to a reluctance to adopt.
This suggests a further objection to the student's thesis. Suppose that, leaving moral considerations aside, a couple honestly ranks the three options as follows: (B) having our biologially own child, (N) having no child at all, (A) having an adopted child. In this case, the student's thesis (if you will have a child at all, then it ought to be an adopted one) may seem to lose its grip. These parents will have a child only if it is biologially their own. By having such a child, they are not withholding the benefit of adoption from any existing child because, even if they had refrained from conceiving, they would not have adopted anyway. It's hard to see, then, why we should follow the student in holding that (N) is morally permissible but (B) is not. This tension could be resolved by weakening the student's thesis to saying: Those who do not have a genuine preference for (N) over (A) ought to adopt a child. Or the tension could also be resolved by strengthening the student's thesis to saying: All couples, even those who prefer (N) to (B), ought to adopt a child.
As you requested, I have given you some counters to what you, like my student, believe. But these counters must, of course, be balanced against the huge benefit that adoption would typically confer on a child in great need. All things considered, many of us may really have strong moral reasons to adopt in preference to conceiving a child of our own.