I am a midwife and trade unionist. I never assign sex and find parents are quite capable of identifying their baby’s sex. My job includes documenting sex in the health record and sex is recorded in birth registration. Sex is only ‘assigned’ to babies when this is not obvious at birth – a rare occurrence. Recording of the sex of male and female newborns is important. There are health issues for newborn boys (such as undescended gonads) that do not exist for newborn girls (for whom internal gonads are healthy) and vice versa. Recording sex at birth, and various life junctures, is vital to the social and political health of us all. Every single claim that feminists and socialists have ever made about the representation and treatment of women is dependent upon this data. This includes that 100 million women were missing due to sex-selective abortion, infanticide and unequal treatment of girls first published in 1990.  Our efforts for a more just, equal and peaceful world starts with being able to accurately identify inequality, injustice and violence. Without recording sex we would not know that even more women (estimated 117 million) are missing today.
Bridget Chapman and Kirstie Paton write that self-declaration will not render meaningless sex discrimination legislation or the category of woman. Yet, Stephen Whittle, a key activist for the Gender Recognition Act described how in that legislation
“gender identity transforms legal sex…there is no recourse to the sexed body which suggests that the body’s sex as a taxonomical tool has in some way become redundant… Changing sex for the purposes of legal recognition then, is … about changing how sex is legally defined.” 
The Equality Act placed limits on this disregard for the sexed body, setting out exceptions in Schedule 3, Paragraph 28.  These allow the provision of single-sex services provided they are a proportionate means of achieving a legitimate aim. The passing of a self-declaration law together with a removal of these exceptions would see the current delicate balance of rights between ‘sex’ and ‘gender’ in the Equality Act overturned.
Socialists who want ‘the body’s sex as a taxonomical tool’ made redundant should reflect on the violent exploitation of female bodies. Women have won services and organisations that meet our needs for safety, dignity, privacy and healing at times when our bodies and psyches are vulnerable.
Those confident that new legislation will not impact women’s rights should not fear full discussion of the proposals. The left must consider the impact of further changing how sex is defined.