"Most trans girls don't play sports anyway"
Trans girls should be able to play in girls sports because trans girls are girls.
"Trans girls should be able to play in girls sports because most of them have low testosterone anyway"
Trans girls should be able to play in girls sports because trans girls are girls.
"Trans girls should be able to play in girls sports because sports shouldn't be segregated by gender in the first place"
Trans girls should be able to play in girls sports because trans girls are girls.
IT IS NOT ABOUT SPORTS, IT IS ABOUT TRANS GIRLS BEING GIRLS.
Find-and-replace "trans" with any descriptor you want and it becomes immediately clear how the issue is transmisogyny and folks not believing that trans girls are girls.
Imagine saying
"Brown haired girls should be able to play in girls sports because they don't have higher testosterone"
That doesn't matter because they are girls!!!