
This is the standard definition of sex used by biologists. Sex is a reproductive role, the genotype just guides the development of that role. In some species like hogfish, their sex changes throughout their lifetime. We know this because their phenotype changes from being organized around the production of ova to being organized around the production of sperm.
Intersex people are either male or female. I think you mean to say a single functionally hermaphroditic person. I don’t think the argument works though. It’s generally accepted to say that humans have two legs, despite rare exceptions existing. One person in several billion would hardly change the accepted fact that sex in humans is binary. It’s a moot point though as no such person has ever been documented.
One person in several billion wouldn’t change a basic fact of human biology. Just as it’s accepted that humans have two legs despite rare exceptions existing, it would still be accurate to say that human sex is binary. This is a moot point though as there are exactly zero confirmed cases of a human being functionally hermaphroditic.