I understand your argument. I don't agree with it, but I understand it.
First, for those reading, it should be noted that arth is not advocating sterilization if you simply don't have the genes--merely if you'd otherwise have been removed from the gene pool. That being said, I can't deny that it still smacks of 'genetic hygiene' programs...
But since you seem to be approaching this from a fairly purely logical standpoint, I'll dispense with the humanitarian side for the time being. The flaw I see in your argument is this: evolution is dumb. Which is to say, if you look at history, it's full of deadends, missteps, and mass extinctions. It is undirected. Unless you believe in ID, in which case, the appeal to natural selection seems unwarranted. One thing seems pretty clear to me, at least--in the general case, genetic diversity in a population tends to be significantly more advantageous than any single mutation when it comes to survival of that population. What is an advantage in one situation may prove to be fatal in another. Sickle cell anemia helps ward off malaria. Myopia mitigates age-induced presbyopia.
Example: a disease comes along and wipes out the part of the population that does not have the CCR5 receptor. The population then uniformly has no CCR5. If another disease were to come along that the CCR5 receptor were to play a significant part in fighting (which is not implausible), then that disease might rampage through the population unchecked, wiping it out completely.
I don't know about you, but personally, I would prefer that my species--or its derivative--survives. If I can weight the dice thus, I will. I don't think natural selection is anything holy, that we're not to stand against. If you, on the other hand, only care about the 'Law' of natural selection and evolution, then I don't think we have enough common ground to establish any sort of framework for discussion. Me, I've got a vested interest.