&gt;  There has to be a way of separating men and women in sport

The whole point is that there has to be a line somewhere* and no matter how we define this line it is somewhat arbitrary. Someone is going to get screwed when they fall on the 'wrong' side of the arbitrary line.

Male and female divisions seem simple enough, but they're really not as pointed out by a lot of comments in the thread. It seems sensible to focus on what causes the actual performance differences, not things like what is between your legs, or gender identity, or any of that.

So, draw an arbitrary line around testosterone seems like one of the 'fairest' ways to resolve the issue. Unfortunately she falls on the wrong side of the line.

It's a bad approach (for her) but unless there is some better way...?

I mean compared to me Usain Bolt is slightly more genetically gifted when it comes to sprinting. He was 'born that way', it's his natural advantage. Caster was 'born that way' so it's reasonable to say it's her natural advantage, but it gets back to deciding on what determines the arbitrary line about who can compete in which division. She's a woman AFAIK, but has one very man like trait - high testosterone. 

*assuming we don't just want men winning at most sports