“Men” and “women” are gender identities. XX and XY chromosomes are genetic traits that inform but do not exclusively prescribe biological sex.

Equating gender and sex is a common tactic used to invalidate and illegitimize the identity of transgender individuals by ignoring their identity (which is the part that actually matters) and focusing only on their biology.

For example: Telling a trans man (born female) that he is a “woman” because he has a uterus is rejecting and denying his identity despite presenting and acting masculine in every other way. Biologically, he’s female, but that doesn’t demand that he acts like a woman.

For an analogy, consider expatriates. Suppose some guy was born in the United States, and after his service in their Veitnam war, he chose to emigrate to France and rescind his American citizenship due to ideological differences with his former country. He’s French by every definition other than his country assigned at birth. You wouldn’t call him American because that’s what he started out as, right?