[lagadu]

Having and X or Y (or even multiple of each, as some people do, XXY and XYY are real, even single X or Y too) is binary, what's not binary is how it doesn't perfectly correlate with the sex (not even getting into gender here) that the person in question belongs to.

[Polizeiposaune]

Single Y is a lethal mutation -- there are too many genes needed for normal development that are present only on the X chromosome.

[andoando]

Sure what I was responding to is "Nothing in biology is really binary anyways". Certainly not everything is binary, we don't have two simple models of all people despite nucleotides having binary pairs, but there is a lot of damn binary in biology.

[EnergyAmy]

XXY and XYY and whatnot are all variations within a sex. Sex is defined by gamete size, females produce the larger gametes and males produce the smaller gametes. In humans, this is binary and immutable.