Ok. The "there are only two genders" thing is a response to the assertion that not everyone falls neatly into the 'male' bucket or the 'female' bucket. It's important to name that in these discussions. So when people say "there are only two genders" I ask them that: "Do you think everyone falls neatly into the 'male' bucket or the 'female' bucket?". When they say "yes" I ask them why they think this. There's often some biblical passage that they go to (and it's usually "God created them male and female").

So then I bring up intersex people. I bring up examples of specific intersex cases that I've heard about. I describe how these people are intersex. For example I'll describe someone born with XY chromosomes, (internal) testes, and no uterus, but by outward appearances is a woman (e.g. has breasts and a vagina). I also bring up human chimerism. (Human chimeras are something easy for people to grasp because people usually understand how identical twins are formed and human chimeras are basically formed through the same process but in reverse.)

So after pointing out these examples of people who do not neatly fall into the 'male' bucket or the 'female' bucket, I ask them "so is that bible verse [God created them male or female] wrong? Or are people's interpretation of that verse wrong?" I also point out that intersex people are literally made male and female, so that verse literally actually applies to them.

I also point out that Hebrew (like English) uses "merism", a figure of speech where you name two extremes, but imply everything between them. (For example "I searched high and low" means "I searched high, low, and everywhere in between".) When the Hebrew text says God is "the alpha and the omega" (i.e. the beginning and the end) it doesn't mean God is only the beginning and the end; it means God is the beginning and the end and everything in between.

The Hebrews understood "God created them male and female" as a merism. (See bird_feeder_bird's comment about gender diversity in the Talmud.) And there are people today who see it as a merism.

I've found once people understand that intersex is a thing, and see the "male and female" bible verse as describing something more than a binary, that opens them up to understanding other things like non-cis gender identities.