*backs up several steps in the discussion*
The woman in question got herself sterilized so she wouldn't engage in the 'vanity' of creating a 'purebred human'.
Even assuming that the uppercrust and nobility are considered to be 'purebred humans', sterilization is a pretty drastic way of preventing herself from having a child with the uppercrust and nobility - i.e., creating a 'purebred human'. There are plenty of people outside that group that would be far more likely for her to meet, much less have a child with.
Now, if she'd said she didn't want children because she didn't want to add to the overpopulation problem, increase her carbon footprint, or just plain didn't want any, that would have been a completely different situation.
Genetically, a purebreed is an organism that produces offspring like itself - like red roses or cocker spaniels. If you mate two cocker spaniels with each other, you get a cocker spaniel, which has certain breed characteristics, not a cockapoo (which would involve a poodle in there). Anything else, from 'tea roses' to most of the vegetables in the supermarket, to the couple down the street that has a blue-eyed daughter and a brown-eyed son is a hybrid.