Why do people say is wrong because of potential for genetic abnormalities in offspring?

I read somewhere recently that the testosterone in mans saliva will release chemical to woman (while kissing) and her body would know if his genes are compatible to hers, if not compatible, (like a brother or close relative) there will be no desire for her to have ual relations with the man. I thought that was interesting because it would stop any because the female would have no desire for , this of course would not apply to . I wish someone would ask this question here because it sounds interesting about the testosterone in mans saliva and compatible genes.