I've been reading the documents that come with this bill.
This is from the Policy Memorandum (PDF):
Sex
15. There has always been a question on sex with a binary response option (male/female) included in the census. The 2011 Census recognised that society‘s understanding of sex has changed and guidance provided explained that the question was being asked in terms of self-identified sex. Looking forward to 2021, consultation has identified the need for a more inclusive approach to measuring sex. The sex question being proposed for the 2021 Census will continue to be one of self-identification and will provide non-binary response options. Importantly, the sex question proposed will not seek a declaration of biological or legal sex.
It was clear watching the evidence sessions that the guidance in 2011 was on display in the bottom of a locked filing cabinet stuck in a disused lavatory with a sign on the door saying ‘Beware of the Leopard. (h/t Douglas Adams)
Yet another thing that's been pushed through quietly, under the radar 
I want to see the people who designed the 2011 census giving evidence to this committee. I want to know who advised them, who and how they consulted. I want accountability.
Meanwhile, in 2011 a question on sexual orientation was not included (despite being a protected characteristic in the shiny new EA) because, the level of public acceptance of the question was not considered sufficient to merit its inclusion in that census.
This is how they define 'sexual orientation' in the policy memorandum:
Sexual orientation is a combination of emotional, romantic, sexual or affectionate attraction or feelings towards another person. How a person determines their sexual orientation can be based on any combination of the above attractions, feelings or behaviours. It can change over time and a person may not know what their sexual orientation is.
I can't quite describe how cross I feel right now.