These debates get complicated really quick, so if you want to counter it, you have to keep your message simple. Part of the reason these anti-biology denialists get away with this is because the word "sex" has so many connotations, so it's easy to confuse people.

1. "Sex" in this context is about the reproductive process. And that process is a fully binary system in humans (and you have to go VERY far away from humans in the animal kingdom to find any exception to this). Either you are equipped to produce an egg, or fertilize one. Whether that equipment is fully functional or not is irrelevant. There is no "third sex" involved in reproduction, nor has any human being ever been a true hermaphrodite, able to both produce eggs and fertilize them. "Intersex" describes a wide variety of conditions, but intersex people all have a sex. And that's not the same thing as transgender anyway.

2. So what is male and female? If you have the SRY gene, you are a male. If not, you are female. It's that simple. Whether you prefer wearing pants or skirts, wearing lipstick or watching football, etc, is irrelevant. None of that has anything to do with biological reproduction.

Note: when you debate this, you can't say "XY" chromosomes = male, and "XX" equals = female, because your opponent will pull a "gotcha" and point out the exceedingly rare cases of XX males. The SRY gene normally is carried on the Y chromosome, but it can be carried on the X. The SRY gene is the true and most objective definition.