Now you want to talk biology? Ok. In biology, gender is defined by the size of the germ cells. Females produce large gametes - egg cells, and males produce small - sperm. This is a striking dichotomy that biologists find in most multicellular organisms.
The dichotomy between large and small gametes also has consequences for other traits. Males and females have different primary sex characteristics: Large gametes are produced by the ovaries and small ones are produced by the testicles. About 99,999 percent of people are born with either ovaries or testicles, ie a strong dichotomy.
How you want to define it is entirely up to you, and I could not care less. But this is the scientific definition, and according to it, men are men and women are women. So sometimes I say that, and I'm on safe scientific grounds.