But where do you draw the line with natural advantages?
We have 2 classes for most sports based on biological sex. The reason for this is the distinct bimodal distribution of physical traits (height, muscle mass, bone density, VO2max etc) which arise largely due to the effect of testosterone - both at the time of competing and during puberty. So for most people the fair line is biological sex. People with intersex conditions will need to be considered individually but if they were producing male levels of testosterone during and after puberty due to internal testes, then unfortunately even if they were completely unaware of this and grew up as a woman, when it comes to sport they fall on the male side of the line.
I don't think the dividing line should have anything to do with external genitalia - on that point I agree with McKinnon, sporting ability isn't determined by whether you have a penis though obviously that leads me to a different conclusion re who should compete in women's sports.