Even though eugenics is verboten, we practice it collectively by selecting the best possible mates. However, because this practice is an undirected effort, it's free from blame. When a government program shapes the gene pool, it's abhorrent. When individuals do it, it's human nature.
Religion is a vehicle for eugenics. By urging the flock to date amongst themselves, religions ask for genetic isolation and hopefully, improvement. Churches further isolate their gene pools by tacitly block "undesirables" from entering, even if those people were originally part of that faith. In small towns, with few other social groups, being excluded from church is as bad, if not worse, than castration. Sure, religious imperatives go under the guise of people of the same beliefs sticking together, indirectly it all leads to selective mating.