Myth? What myth, exactly? The "myth" of only two sexes or the "myth" of having 5 fingers?Even with No of fingers or other phenotypes an agreed framework would be needed, how much of a stub counts, are 10 stubs to the first knuckle 5 fingers, the beadle is an example!
Frankly a clear determination of how to classify intersex people is not likely widely agreed other than by people peddling your above and clearly flawed myth.
Genuinely intersex people represent the unfortunate reality that things go wrong. People have deformities. People have faulty genes.
That doesn't mean there are more than two sexes. It just means that the process of determining your sex doesn't execute properly in all people.
So what do you do with Caster Semenya, for instance? She's not a woman, she's not a man, she's one of the aforementioned unfortunates.
What she isn't a third sex for our species. She's intersex.
Otherwise you'll need an infinite number of "sexes", just as there are an infinite number of numbers between 0 and 1. Where most males would be near 0 and most females would be near 1, but some unfortunates end up in no-man's land.
Who does it help to have more than two sexes (+"intersex"), btw?