The concerning thing is, whichever way this is explained to the child as they grow up, in whatever age-appropriate ways, it is likely to be telling them something that is a fiction, and outside reality, particularly if the story is 'your dad is a man, and he was pregnant with you and gave birth to you'.
Exactly.
Gay couples don't lie to their kids, they explain things in an age appropriate way. Women who use sperm donors don't (or shouldnt anyway) tell their kids that they conceived them on their own, they explain things in an age appropriate way.
If you want to lie on your own child's birth certificate to validate your own feelings, then how are you going to explain any of it to them when they are older?
Out of interest actually, because I don't know, what does go on the birth certificate of the child of a gay couple?