If it is agreed that gender is different from sex and gender is a spectrum and sex is a biological fact why can't both be put on birth certificates? Sex recorded as it is now, either male or female and can only be changed in exceptional cases and upon medical proof I.e. a dsd as found in intersex people. Gender could be recorded as matching the birth sex but easily changed once a person reaches adulthood at 16 for a nominal fee to cover admin costs. Sex characteristics are protected and gender identity is recognised.