I agree with you. Arguing that it would have been breed out is also ridiculous. If that were true, there would be no children with cancer or other deadly childhood diseases. Just because you don’t get the gene doesn’t mean you don’t carry it.
Having said that, I very seriously doubt it is 100% genetic. My bet would be for males it is 50% genetic and for females 1%.
To sin is a basic characteristic of all men, so I could hazard a guess that homosexuality is something a person could be born with. The thing about homosexuals, is, their propensity towards sinfulness is a spiritual disability, not a genetic predisposition.