Woodifer, I think the fairness is where you put the line. With boxing weight divisions, someobody whose natural fighting weight is near the top of their division has a bit of an advantage over somebody near the bottom of the division, and possibly also someone who is naturally a bit heavier and has had to starve off muscle as well as fat to make the weight. But everyone accepts this is fair because you have to put the dividing line somewhere and it's nice and clearly defined by one easily measured parameter.
Whereas for example in Olympic athletics, the IAAF have been forced by CAS to err far too far towards the male end of the spectrum when choosing where to put the line. By allowing intersex and transpeople in women's sport, I feel that this effectively turns sport into "male" and "other"- as usual- male as normal, default human, woman as everyone left over.
The decision to let people who have never had ovaries, have had and may still have testes with the irreversible advantages (pelvis shape etc) that this brings, into women's sport seems based on a deep rooted sexism to me...using biologically male athletes as the point of reference, saying well their testosterone is 1000% higher than a woman, OK well trans/intersex can have 300% higher testosterone than a woman...still not as high as a man therefore Not-man, therefore woman. I don't believe this definition was chosen to save biologically normal female athletes with PCOS and outlyingly high testosterone from being excluded, I think it was from a sexist way of defining women- ie wholly in relation to men ( and perhaps instinctively not wanting to sully the lovely pure mental image of proper Man with anyone "less"- hence lumping the lesser all together and labelling them all Women.
(OK I concede that my "defining a woman as anything less than a perfect man" theory does not account for FTM people- but they will be vanishingly rare or non existent in the top echelons of men's sport for obvious reasons).