But there exists a “legal fiction” (I think that’s the term) that this person is a man. It seems that birth certificates also contain legal fictions (married lesbians, sperm donors
I know it can seem as though having two women on a birth certificate is a legal fiction, but as the law has never required that a father is named on a birth certificate, and as the second person on the birth certificate has never routinely had to show a genetic relationship with the baby, really the law hasn't really changed that much.
Somebody married to somebody who has given birth will become the baby's legal guardian by default, even if neither of them have a genetic relationship to the child and even if they are another woman.
A birth certificate is a record of birth, not genetics. The only two people who must be on a birth certificate are the baby and the mother, 'mother' simply meaning the person who gave birth.
If the law is changed to say that a father can give birth, it is a fundamental change of the meaning of the words 'mother' and 'father', giving them values that they don't currently have in law.
A birth certificate does not predict whether somebody will be a good or bad parent, or define their parenting role or predict that they will have any long term role in the child's life at all. It is a simple statement of fact. This person gave birth. The word for a person who gave birth is a mother.
Clearly the Guardian journalist has all sorts of issues with the words 'mother' and 'woman'. That must be very difficult. However, the proposed change fundamentally alters the purpose of a birth certificate and we should all have a say in that.