That's not what he is saying. He is speaking in abstractions about what is best for society overall, and what our laws about childbearing and rearing should reflect. He is not rejecting or denigrating the individual contributions of adoptive parents, nor the status of individual adoptees.
Speaking in abstractions is nice, but the reality is that many humans create children that they are unable/do not want to take care of. These children, adopted by parents who love and cherish them as their own, should not be considered any less than bio children.