With selfhood itself at stake, it's no wonder otherwise rational and tolerant people tip toe around the topic. In a sexually reproducing mammalian species like ours, females and males should be able to be defined as such. Sex chromosomes aren't always XX and XY, and birds for example, males have a pair of identical sex chromosomes, Z, Z, and females have two different ones, Z, W. Many species don't even rely on chromosomes to make males and females; in turtles and crocodiles, it's the temperature of the eggs that determines the sex of the hatchlings.