The point about it is that genes do not mutate under environmental pressure during natal development.
Lysenkoism is a cardinal sin in the biology world...although late breaking news is that there might - and only might - be an environmental pressure that leads to genetic mutation by protein modification, the emerging field of Epigenetics.
Doubt if anything as complex as homosexuality would be the result of such a thing.
Jung suggested that homosexuality is an urban phenomenon, arising perhaps from the difficulty of maintaining traditional sexual roles in a very complicated social environment.