I think the intentions are important. If you are attracted to cis women (born w/ vaginas) and you happen to fall for a trans woman (but they're not typically your type), like you're not fixated on them/chasing them, you can be an open minded straight man. However if you are a chaser, only after trans women for their penis, I'd say you're bi. No different than men who are only after cis women for their vaginas.
If the trans woman had reassignment, using my own logic I used earlier with what I'm attracted to, I'm only attracted to the female body and female image, I could only date a trans woman if she is post op (has a vagina now). No penis in my bed, no matter how beautiful she is. I'd say the man is heterosexual without a doubt.
Trans women ironically embody the perfect woman based on societal standards so it's not impossible to comprehend why heterosexual men would be attracted to them. Most put a lot energy into beauty and fitting into societal female lanes. "Shemale" porn is one of the top grossing porn genres and primarily viewed by straight men. If most of them were born with vaginas, men would thirst on them un-apologetically in the open. It's just the fact that these men can't fathom the experience these women have, grow up similar to them and that can intimidate men. I believe the main men who bash them are attracted to them and they hate that they are.