Its really not in this instance it is extremely straightforward.
If you are born a male and transition to become a female you cannot compete in professional/elite level sports against other females.
Its a completely unfair advantage and it should not be permitted. Would you allow performance enhancing drug users to compete? Same answer and for the same reason, "No, because its an unfair advantage".
it's only straightforward to you because you don't care about discrimination against trans people, who are already one of the most marginalised groups in society. whether you agree with transgender women in sports or not, to categorise it as 'straightforward' is just inaccurate.