From the paper:

As we enter this complex conversation, we recognize that binary categories based on reproductive biologies or gender identities may make sense to include in analyses in order to address certain questions in human biology.

So even according to the paper, sometimes binaries are fine. Also, speaking of Fausto-Sterling, it cites her brainrot uncritically:

Although categories may be useful for addressing major issues of exclusion, feminist scientists have critiqued the concept of binary sex (e.g., Fausto-Sterling, 1993)

And have you read her paper?

For biologically speaking, there are many gradations running from female to male; and depending on how one calls the shots, one can argue that along that spectrum lie at least five sexes-and perhaps even more.

There is zero indication that it’s tongue-in-cheek when reading it, it’s been cited seriously in literature such as your link, and a good faith reading of it leads one to think she believes in 5 sexes. I mean come on, this is just nonsense. She’s a clown.

Zachary Dubois has a PhD from the Department of Anthropology but doesn’t list it specifically as a degree in biological anthropology in his CV. I don’t think it’s worth quibbling over whether he “counts” as a biologist, but I wasn’t lying and at worst was too dismissive. Either way, he’s not the person to look to for fundamental definitions in the field of biology.

Do you know what your binary definition has been usefull for? […]

Again, it’s not my definition. It’s the common definition used in biology, and is very useful for science. That some people can misunderstand it and try “fixing” people using faulty logic is immaterial.

And hopefully this helps clear things up for you. From the same author I linked to before (PhD Evolutionary Biology):

Such mixed sex development is exceptionally rare because evolution has ensured developmental mechanisms to make sure this is so. A growing embryo will be wasting resources if it develops organs and tissues that cannot contribute to future reproduction. Novella’s paper on mice (above) is actually about a gene that appears to be involved in cross-sex development suppression. Put simply, our development of reproductive anatomy is absolutely not a pick-‘n’-mix of organs and tissues from male and female parts that might just result in enough of one’s sexed parts to enable an individual to be fertile and reproduce. Instead, it is a tightly regulated cascade of genetic events along a pathway that puts all development effort into male or female development. That is why pretty much everyone ends up as unambiguously male or female even when significant development conditions occur. Male and female development are mutually antagonistic.

Very rarely, and for reasons not well understood, the brakes may come off and tissue development that is normally suppressed starts to grow. It is a bit like a cancer where the normal growth regulating mechanisms fail. And indeed ovotesticular disorder is associated with malignancies of these tissues, so are often surgically removed soon after diagnosis to prevent lethal cancers.

What is not observed is an individual who is fertile both as a male and female. If fertile at all, it will be as one sex. The cross-sex tissue is typically under-developed. No human is a true hermaphrodite (in the biological sense as being able to reproduce as both a male and female). Unfortunately, medicine also uses the term “true hermaphrodite” to describe people with these very rare disorders. Do not be fooled by this equivocation.

So despite this cross-sex development, can we still say what sex a person is? That is a complex question as we are dealing with disorders that are so rare and with so many different causes and outcomes that a blanket statement is not easy. Doctors publish individual case reports where it may be clear a person has undergone predominately one sex development and in which case we may be confident in calling someone male or female. It is a matter of debate if there exist individuals where sex development is so mixed that such a classification is inherently meaningless. But even if some individual were truly sexually ambiguous, they would still not be a third sex.