Why don't we strip this back to the questions:
'what is the purpose of a birth certificate?'
'who is it for?'
Families have changed so much in how they are structured and function (gay couples, lesbian couples, egg donors, sperm donors, out of 'wedlock') I don't think we can look at how things historically have worked, but look at what is needed now.
Thinking of those question, birth certificates are clearly for the child, the parents, and the state.
The parents - to prove legal parentship
The state - accurate/factual record keeping
The child - to know who parents are, and also to know genetics..
Knowing one's genetics is much more important these days, now we know how many diseases are hereditory. To be able to know if you are pre-dispositioned for certain cancers is quite important.
I think that there is an innate curiosity in us to want to know who our ancestors are. This is clear with how popular DNA testing for ethnicity/geneaology is. Similiarly the high use it of ancestory websites.
So maybe updating birth certificates so that they show:
Parental (legal) mother at time of birth:
Birthing mother:
Genetic mother:
Parental (legal) father at time of birth:
Genetic father:
This way it covers all situations... if you were legal, and birthing and genetic mother then your name would appear three times. If another egg was used, then the egg donors name would appear. If a surrogate was used, her name would appear.
If you were in a lesbian couple, both names would appear as legal mother, but only one as birthing and genetic.
It could be that only parents listed as parental/legal have any parental rights. The names listed as genetic and birthing are only their for the child to be able to know their history.
It might also mean that three parents could be listed as having parental rights: maybe a lesbian couple used a friend who is a sperm donor and they also want him to have parental rights. His name could be under legal father, whilst both theirs also under legal mother.
I think what is important is not to allow emotions to get in the way, its obviously an important document for all of us, and in the end it is our own birth certificates which we use throughout our own lives, so what does the child need, not 'how is does this make me feel'.