The Act defines situations in which discrimination against trans people and married people is permissible, but does not do the same for 'cis' people or single people because an exemption is not required (there are no exemptions at all from disability protection).
Alternatively, it does not do the same for 'cis' or single people because it's not permitted.
Can you give me an example where it would be lawful to discriminate against a person for being single?
I have to point out that 13(3) of the Act explicitly allows positive discrimination in favour of the PC of disability. It says If the protected characteristic is disability, and B is not a disabled person, A does not discriminate against B only because A treats or would treat disabled persons more favourably than A treats B.
For every other PC, we understand that (for example) if the protected characteristic is marriage or civil partnership, A does discriminate against B only because A treats a married person more favourably than a single person.
Atkins is just plain wrong on the law.
Again, I remind you that the claim for unlawful direct discrimination stands or falls on the basis on which the decision to provide less favourable treatment is reached; the Act says nothing (other than for disability) in which direction the less favourable treatment is provided.