Mammals are a subclass of animals with vertebrates who have features including breasts, hair, and a neocortex. They are distinguished from reptiles, birds, and other vertebrates.
Studies have shown there is no single determinant of sex, gender, or sexuality. In terms of both sex (genetic) and gender (a social construct), a binary distinction of “male or female” fails to describe a wide range of humans in practice. Meanwhile, sexuality (a preference) eludes any simple categorization as well. Simply put, despite some binary aspects (for example a person either has a Y gene or they don’t), Gender, Sex, and Sexuality are all non-binary and each is instead a complex spectrum… and we can prove it with science and logic.
Nettie Stevens and Edmund Beecher Wilson independently discovered the chromosomal XY sex-determination system (that males have XY and females have XX sex chromosomes).