The ideologue is basically right. But in biology there are no rules, only regularities, and evolution tends to explore every conceivable exception to those regularities. Some exceptions are fatal or don’t reproduce effectively, but that doesn’t stop them from spontaneously reappearing. You can just read the Wikipedia articles on sex determination and see most of the exceptional situations.
I deal with fungi, which is a kingdom where many of the most spectacular deviations from M/F appear. Species with over 200 sexes, clonal species with effectively no sexes, and species where cells are two compatible sexes at the same time and can go through the whole sexual chromosomal routine all by themselves.
I don’t like how he goes "the exception proves the rule’. What rule?
That apparently X & Y chromosomes are the most important factor in sex determination? There are genes on the other chromosomes that act as effect modifiers.
How do you argue against the idea that sex is binary
Ignoring chromosomes being the basis for the argument, and opening the floor to a more general context instead that is less restrictive and less biased - the idea that sex is binary requires that there are only two sexes possible in a system called “Sexes”. If there is at least a third sex, then the statement “sex is binary” is false. We can introduce non-sexual organisms to pose as a third, but as they would be non-sexual, they would be disqualified from the binary sexual system and can be ignored.
So, let’s talk binary systems. There is always room to speculate that there is a lot that can be possible between A and B, or 0 and 1, or X and Y. We do not live in a binary reality, though a lot of reality does make use of binary systems so that we can distinguish similar things that would otherwise appear exactly the same. There is always a range between two plots or points on a graph. This range is a length from anywhere between 1 to infinity.
So, even if sex is binary, everything that is a result of the binary system of sex, or is what remains between the two sexes, would not be binary at all. There are an indefinite, infinite and immeasurable amount of sexes.
However, back to the sake of speaking Human terms, things are a lot more convoluted. Humans have their own subset of particular definitions and classifications on what is and is not. Some are emotionally backed by strong internal reasoning, some are scientifically backed, some are backed by authoritative principle, and others are randomly assigned. Good luck. Here is where my journey ends, as I am not too excited to go weeding through the blob of human-orchestrated complexities assigned to things that are otherwise very simplistic at the surface, in order to find a specific needle in a factory of haystacks - a specific pin to use to pop the argument bubble that sex is binary.
Nonetheless, this is a valid answer. Whether it is helpful, is entirely subjective.
What happens if they introduce gametes into the equasion? They think that the body is meant to produce either ovid or semen and that the body forms around that…but that just determines what gametes you produce and says noting about the rest of the body plan, not to mention that it doesn’t determine the validity of your body or identity.