Binary is not a natural phenomenon. Its a constructed model simplifying complex systems and data sets into approximations to two possible values. The underlying data is never identical and nature doesn't use numbers which represent discrete values. Nature is full colour glorious reality.
So if you accept sex and gender to both be social constructs then they can indeed be binary, but the scientific evidence if critically analysed does not support that view. Dealing with chromosomes as an example simply because they are scientific fact as opposed to identity which is a squishy amorphous undefined idea not easily quantified (how we can then make that binary i have no idea)
We have people here saying that atypical chromosome combinations are disorders of the binary xx and xy. What their arguments do not seem to take into account is they are applying their binary model to the data set in order to prove the binary model is true. This is not very sound science but its not really a surprising result. The binary sex system has existed for 1000's of years before we discovered chromosomes, it is not unexpected that when we discovered that there were more things that define sex than genitals, that society forced its well established model onto the new data and consequently made the approximations that are key to forming binary systems and labeled all those variations as disorders of the established model, which is entirely unfair on those who they represent.
If today we were to throw away all historical baggage and come up with a model of sex and gender based on the scientific knowledge of humans we have today, i'm pretty sure we would come up with far more sex and gender descriptions than two. The chromosomal combinations would force it for one, before even getting into identity and ones sense of self.
Indeed there are many cultures of humanity which do not recognise the binary sex and gender system and those cultures have the language to describe the extra variations.
Our historical western christian society which was going out into the world conquering new territories and needing populations to fill those continents were not accepting of non procreating behaviours and as such suppressed homosexuality and gender variance in our society and often those we conquered too, something we are still struggling with to this very day.
So no sex and gender are not binary, but the model we hold onto with vicious doggedness is, and at the detriment to some of those forced to live within it.