Who except a farmer is ever in a situation where getting the sex of a sheep right actually matters? I'm sure if you put a sheep on sex hormones and gave it some surgery you'd convince me to answer the other way, and I'm sure as fuck not chromosome testing the sheep to find out.

So is it based on genitalia? Is it chromosomes? Is it what your birth certificate says? How does any of this affect intersex people? How do you intend on actually checking without it being both ludicrously impractical and invasive? And if you say "I can always tell," no, you can't. You do not know when you can't tell. That's what not being able to tell is.