Well it is a question of definition. If people define gender to mean "a specific category of how you define yourself sexually and what you like in a sexual way" (I'm sure that's not what the oxford dictionary would ever say, but just bear with this oversimplified examples) than that is what it means. That's the purpose of a definition.
Séx however is defined biologically and biologically there are 2 sexes. (And then there are people who have an opposite second chromosome to how they look, but that is about it.) This view, that biology actually tells us some truth – that is not just a human construction some backwards person came up with – is nowadays called: Biological Essentialism.
And if you hold it, you are in grave danger of having an angry twitter mob show up at your twitter-doorstep. Basically you'll get a bunch of easily offended and triggered people who don't like that nature is what it is and that they cannot infinetely mold it – and themselves – to their own image of what is good and right or what they currently like or dislike. And you'll have to put up with that, because in this time and age they have more power than the rest of us.
But, the rest of us are way more people than them. We'll just all need to get up on our feet and tell them off. If enough people do that, all this mess will resolve itself.
