... Biologically, he's still 100% female, just as a lot of real world transsexuals are. Sex isn't gender, and sexual orientation is what it says on the label. You can be a straight male and interact with a trans male without suddenly being gay, if that's what scares you. ...
Not that I know my way around this topic all that much - never played a role IRL for me and I only stumbled about it/started thinking about it because of WVM/Apartment 69, but it's even more than that I think.
(If I'm ignorant here, please tell me, it's not on purpose!)
It is something that was mentioned either in one of these 2 threads or a youtube video linked from them.
Attraction - at least initially, is mostly based on external stimuli due to the lack of others. If you don't know a person, but feel attracted to them and they 'seem' to be female, chances are high you are straight in case you identify as a man and vice versa. (Otherwise, traps wouldn't be a thing.)
So, walking past Skye and being attracted to him doesn't make you gay if you are a guy.
After getting to know a person, it gets less clear at first. But unless you are attracted to the concept of Skye identifying as a male - a tomboy also has 'male traits' - are you gay in case you are attracted to one?
Ignoring everything but 'external stimuli', I'd say if you are attracted to Skye, knowing him, but not Shauna as a trans woman - that still sounds like being attracted to feminine stimuli.
Same if you are attracted to a trans woman Shauna, but not her dick (so her feminine face/boobs/...).
I'd even say, being straight/gay is out the window as a classification outside bodily/external attraction.
Without an attached body, how do you differentiate minds/feelings/views in terms of gender or sexuality? Sounds pretty problematic, starting with the tomboy example.
Then, what difference does it make if that body was grown or altered into its present form if its mere presence triggers (non-)attraction already?
Which in turn makes it pretty easy again I think. Based on what you identify as, what are you attracted to? The male body? The female body? Both?
This is somewhat simplified/boiled down, but as I said, I feel like straight/gay are worthless beyond physical attraction.
We could start, for example, 'what if a man and a pre-op trans man are together and he gets the op?'
But then, either they talked about it so the man is probably at least bi.
Or, the trans man didn't tell and it comes 'out of nowhere'.
Letting aside that that is shitty behavior in a relationship, and on its own might cause a break-up, I can't see the man remaining in the relationship for years/decades/... if he is not at least bi. If he was straight and not at least open to trying (bi-curious?), how should that ever work long-term? So they would most likely break-up if he was straight. But might very well stay friends - potentially? At least there would be no problem 'with the ex' then.
So the same as breaking up with a woman because it doesn't work, but staying friends because you still connect/share something - just without later complications.
If the trans man stays pre-op - from the man's point of view, what is the difference to being with a tomboy/female athlete? Does the usage of 'he' regarding your partner make you gay? Does the sexual orientation of your partner in conjunction with their mere identification (without their physical body) influence your own sexual orientation? I don't think it does.