You are misunderstanding my assertion. When I speak of sexuality and gender identity, I mean within the same person. Let me use myself as an example.
I have always been attracted to men and have never found any desire for women. The fact that my gender identity changed from a tomboyish female to now identifying as male, has not changed my attraction to men. Before I was seen as straight, now I'm seen as gay.
The reason why this matters is because after I came out as trans, many people assumed I would remain 'straight' and because of that, they also assumed I now found women attractive. My transitioning from female to male does not change who I am attracted to. I am still very much attracted to men (and honestly I don't really care if men are attracted to me or not anymore, so people using that as a threat against me doesn't work).
Yes - I know the whole biology argument, I can't change my chromosomes or biological sex - but I'm not talking about that here.