Right, but geneticism is only ever applied to the controversy re: orientation and gender identity in the context of choice. Its importance in this debate is purely to rationalize "wrong" behavior - and thereby cement the belief that it
can be wrong.
Notice that arguments that homosexuality is genetic is preceded by arguments that homosexuality is a choice. Instead of defeating the thoroughly shitty notion that homosexuality is inherently wrong, the argument is instead steered towards "yeah, but they can't help it".
Without the choice angle, geneticism would just be a footnote. Whether or not orientation and gender identity are genetic is entirely irrelevant to the morality debate, and its presence in the debate predicates upon the assumption that, if it weren't genetic, the morality of the situation changes.
This stretches beyond the sexuality debate, of course. Whenever the argument is made that we shouldn't discriminate based on things people can't change, the implicit baggage argument is that if we could change those things, we should.
The correct rebuttal is "there's nothing wrong with being gay/bi/black/yellow/red" not "but they didn't choose to be gay/bi/black/yellow/red".