This may be sort of necro posting (a week), but I disagree. For one thing with the topic title, it is definitely not natural selection. As for reverse, it's just not. The idea of natural selection is that the genes of the un-select do not procreate, which is the exactly the suggestion you propose. I disagree that natural selection is unable to occur. Natural selection only occurs under two conditions: need and random mutations. Without need, the beneficial mutations will not be spread through the populace, but they also do not go away. So, if the need should arise, there will still be the lucky people who have genes to suit the need. This is not affected by the existence of people who do not have the genes that will help them survive should need arise. Natural selection does not kill off the weakest in every generation, as you seem to suggest, but rather it kills off the weakest when need arises, and the weakest is determined by the need. Also, you seem to be implying in your argument that "mentally handicapped" by neccesity makes them weakest. This is only the case in a world which rejects the infirm: i.e. a world that would be created were we to sterilize them. Should a need arise for natural selection (say, a disease that spreads rapidly), it is just as possible for a mentally handicapped person to develop an immunity as it is for any other person, and in that case, the mentally handicapped would become the fit, and regular people the weak.
They are not really corrupting the gene pool, as there are not enough of them to do so, and furthermore, most mentally handicapped people are born from regular parents, and their handicaps are not genetic, but results of things which happened during their development. So, were they to procreate, they would not pass on a harmful gene. We will not continue in a spiral of deformity. I assume you are referring to lesser intellect as the deformity, but if anything, humanity has gotten succesively smarter with each generation. Also, there is no such thing as de-evolution, and deformity is not a natural term. If some disaster caused the forest to turn white, then albino animals be the 'select' and other creatures will be the ones to die. Deformity is situational only.
On a moral level,I believe it to be wrong to define a class of people as inferior and to prohibit them from reproducing because they belong to that class. Despite a lack of intellect, the mentally handicapped are still people, loved and capable of loving: and denying them the ability to have children is reprehensible to me. So I disagree with you.