Please don't tell me I'm wrong, when you are using terms like 'stock' to describe genetic diversity. Eugenics is the societal level identification of desired traits, and the subsequent attempts, again at the societal level, to select for those desired traits/select against undesired ones. This isn't debatable, this is a textbook definition.
Disallowing incestuous relationships hasn't been driven by any sort of wish to alter the gene pool of a society historically, as far as I'm aware. You can argue that these relationships should/shouldn't be allowed for whatever reason you want, but saying eugenics is bad, and disallowing incest is eugenics, thus disallowing incest is bad, is flawed logic, because it isn't eugenics. Assuming the incestuous partners were carriers of the alleles for X undesired traits (and assuming this was identified somehow), eugenics would forbid them from mating with anyone, not just each other, since the purpose is the elimination of the trait (which genetically, means the elimination for the allele that contributes to the trait), not just preventing affected individuals.