By treating genetic disorders, natural selection is interrupted -- these individuals do not die as they naturally would have, and so their genetic disadvantage no longer selects against them. iT could be argued, however, that humans have stopped evolving as biological creatures anyway; technology has provided the "cure" to many issues of natural selection, both from the species end of things and from the supply side (i.e. In making more resources more available to more people). Therefore, it is not really detrimental to the species as a whole to save the individuals with lethal alleles. Since we are no longer really evolving, and the prevalence of most lethal alleles is incredibly low anyway, the species as a whole is not made less healthy by the presence of these individuals or their alleles, despite the increased chance they have at procreating.
The only real issue that could conceivably arise from this requires a serious "what if" scenario. It is possible that at some point in the future of human civilization all medical and technological knowledge will be lost. If that occurs after many generations of caring for those with genetic disorders in ways that allow them to procreate (and that are not gene therapies that effectively remove the alleles from circulation), there is a possibility that enough genetic load will exist to affect the species stability. Again, though, given how rare such alleles are it is unlikely that this would ever get to be a problem.
