Will be interesting to read the detail of the SoS's Statement of Reasons, due to be published soon.
ETA: here it is (not read yet):
The policy rationale considered by the UK government leading to its decision to make an order under section 35 of the Scotland Act 1998 preventing the Gender Recognition Reform (Scotland) Bill from proceeding to Royal Assent.
www.gov.uk
ETA: now read it. In summary, the claim is that the bill would have adverse effects in three ways:
1. As a result of differing regime across the UK. This is the idea that it would result in people having a different sex either side of the border, making cross-border stuff e.g. UK-wide clubs or enployers unworkable.
2. Because of the increased risk of fraudulent applications - primarily that this will deter women from using single- sex facilities.
3. Because of adverse effects on the operation of the Equality Act 2010. These can be further divided:
a. Exacerbating of existing issues including regarding single sex clubs, public sector equality duty, equal pay, and the use of single-sex exemptions.
b. New issues - essentially problems for single-sex schools.
Some are arguable; others seem misconceived. Will be interesting to see how it plays out in The Supreme Court if the SoS's decision is subject to a claim for Judicial Review.
ETA: 3a is quite telling- the idea that the current system is problematic in practice.