I do think the EA is a bit weird with the protected characteristics. Why isn’t pregnancy and maternity covered by ‘sex’? If marriage and civil partnership are essentially about not discriminating against civil partners over married couples, why isn’t it covered by ‘sexual orientation’?
’Gender reassignment’ is covered by ‘sex’, ‘sexual orientation’, ‘religion or belief’ and it could be argued ‘disability’ because of the high prevalence of ND, so why was GR added when there is no verifiable definition?
This all seems to be a very confused understanding. If a woman is discriminated against because of pregnancy, then the protected characteristic of sex ie a result of a woman's biology is covered. Who told you it wasnt.
Not all civil partnerships are same sex, so not clear why you thought it should be about sexual orientation?? Are you aware of why people of all faiths, sexual orientation campaigned for civil partnerships. Anyway it is a redundant arguement as there can be same sex marriages, which is more relvant to why the GRA hasn't been got rid of as it was primarily to allow same sex people to get married at a time marriage was only for people of the opposite sex. So a GRA meant that one person became "legally" the other person for the purpose of marriage.
Gender re-assignment is covered by gender reassingment (ie one of the actual protected characteristic as some think it is gender identity!)
The only legal interaction between the protected characteristics of sex and gender reassignment are the SSE.
Have you read the legislation (rather than a stonewall influenced briefing)?! https://www.legislation.gov.uk/ukpga/2010/15/part/2/chapter/1