Call me a wishy-washy fence-sitting liberal-with-a-small-L, but I think this is just a job he and colleagues have been tasked with - to get a legal definition that will stand in all cases.
Otherwise, it'll be open to definition/challenge/appeal at every turn.
We can only hope that biology and common sense prevail😕
Lord Reed presents himself as a no-nonsense, modern kinda legal eagle, but part of the 'modern' bit is saying that the law needs to keep up with changes in society, and that's worrying.
On the other hand, he may want to make the law more straightforward, and he may not want to build in new legal fictions, e.g. a man with a special piece of paper is a woman. Sticking to XX-XY would take a lot of the hassle out of your decision-making process, m'luds and ladies!