You're right there are other combinations due to abnormalities, but those are rare and don't a lot of them involve ambiguous genitalia? I don't know, tbh.
As for what it adds to a birth certificate- the whole point is to track genealogy and family lines and keep such records for whatever purpose (tax records, etc). The govt. might not care if you have a male or female baby, but if you neglect to include that information on a birth certificate it can create confusion down the line when you're trying to sort out who begat who. There are plenty of gender neutral names, Leslie for instance, and nowadays more and more names are crossing the line into neutrality. So is this Leslie a mother or a father?
It's also just one more way of helping to visually identify someone, especially when they don't have a photo ID (first time getting a license, or whatever). They have to bring in more than one form of ID, and if someone waltzed in with boobs, high heels and a purse and said "My name is Alexandra", but the birth cert said it should be a male with the name of Alexander, what would you think? Transgender or fraudster? And how are you going to determine the difference? There is no real way to do so.
So I am still in favor of making supplement forms to help alleviate the issue. Rather, I certainly wouldn't oppose it if it came up.
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