They've actually done medical studies that prove a man can be straight and be attracted to a trans woman. Heart and brain monitors along with blood flow monitors on the penis. They monitored the data while showing nude pictures of all kinds of fit people. Men, women, and trans men and women. Straight men were turned off/repulsed when seeing other naked men or gay porn. Those same men would get turned on with naked women pics or trans women that actually had feminine looking bodies..... even if they had a penis.
It seems it's more about the overall physical appearance than the genitalia involved. Perplexing and IMO, why so many people despise trans people because they're worried that they'll be "flipped" by one. Not that it would even matter unless they want kids. It doesn't mean they're gay......and that doesn't even matter either.
Also considering the way medical science is evolving it's only a matter of time before 2 people of the same sex can have a baby that has both of their DNA just like a straight couple. It IS going to happen no matter how much some are opposed to it.