we talking a man transitioning to a woman or a woman transitioning to a man?
either way, the answer is they wouldn't make a roster due to the hormones required to either reach their desired outcome. a woman transitioning to a man would never be athletic enough, but even if they were they'd be on PEDs to do so. a man transitioning to a woman is generally take hormones in the opposite direction.
there are plenty of athletic gay dudes who been in all the leagues though. that shyt ain't the same.
