This is interesting.
On the surface, I had a somewhat dissmissive reaction to your primary claim. But, the breakdown starts to make sense, especially the reversal of (B), as it is so obviously the case. I don't have much knowledge of (A) or (C), but it sounds plausible that you could take a tomboy or a feminine boy in the confused state of adolescence, and manipulate the inputs to nudge or even change their preferences and behavior.
If you remove the hotbutton context of homosexuality, I would be hard to argue that this level of manipulation isn't possible. Or, that if a resourceful and organized force which had this as a global goal, and control of the media, could not achieve it on a perceivable scale. I believe homosexuality is estimated to be around 3% of the US population. Trans is growing quickly.
My instant reaction makes me wonder to what level I have been socially conditioned around the topic. As someone who considers themselves a skeptic, it is easy to then extrapolate to people who have no interest in skepticism and to what their level of conditioning may be. And, how reasoned discussions around the topic are almost impossible.