The claim I'm defending is that it's "multidimensional", by which I mean "many dimensions", not just two.
Somewhere I started making a list, but I've no idea where it is at the moment, so this is just off the top of my head:
The basics - physical gender, sometimes confusingly called "sex": chromosome configuration (XX/XY/other), genital configuration (not always clear-cut), chest configuration (including functionality of mammary glands), amount of subcutaneous fat, skeletal shape (hip size being the most obvious detail, but there are others), hormone levels, facial and non-pubic body hair amounts, physical strength (especially upper-body).
Behavioral characteristics: "girlyness" (e.g. liking flowers, ponies, kitten) vs. "manlyness" (e.g. liking motorcycles, trucks, guns), passivity vs. aggression, emotional vs. analytical, cooperation vs. confrontation, empathy vs. control, nurturing vs. self-interest...
Attractions: the main two dimensions here are whether you're attracted to men and whether you're attracted to women. (There's a high degree of inverse correlation, and most male-bodied people are attracted to women only, but I should think that the obvious exceptions to this rule alone would prove my point.)
I could go on, but that's the stuff that's easier to explain.
Re: opinion contradicted by evidence
Somewhere I started making a list, but I've no idea where it is at the moment, so this is just off the top of my head:
The basics - physical gender, sometimes confusingly called "sex": chromosome configuration (XX/XY/other), genital configuration (not always clear-cut), chest configuration (including functionality of mammary glands), amount of subcutaneous fat, skeletal shape (hip size being the most obvious detail, but there are others), hormone levels, facial and non-pubic body hair amounts, physical strength (especially upper-body).
Behavioral characteristics: "girlyness" (e.g. liking flowers, ponies, kitten) vs. "manlyness" (e.g. liking motorcycles, trucks, guns), passivity vs. aggression, emotional vs. analytical, cooperation vs. confrontation, empathy vs. control, nurturing vs. self-interest...
Attractions: the main two dimensions here are whether you're attracted to men and whether you're attracted to women. (There's a high degree of inverse correlation, and most male-bodied people are attracted to women only, but I should think that the obvious exceptions to this rule alone would prove my point.)
I could go on, but that's the stuff that's easier to explain.