They are irrelevant, since they don't affect the interests that various individuals have in life, pleasure, the avoidance of pain, etc. Singer summarizes a number of scientific findings to show there probably are significant differences between males and females. He stresses, however, that the inequalities actually inflicted by society on women are wildly out of proportion to those differences. Consider genetic endowment, environment, education, family income etc. Equality doesn't mean it's wrong to reject someone who's short, or poor at physics, if I need an employee who is tall, or able to do physics.
Affirmative action seems to flout equality.
Mere disproportion of members of a certain group doesn't prove discrimination. Disadvantaged people scoring lower may thereby show higher ability. But the clinching argument is that university admissions aren't based on consideration of the applicants interests, but of the institution and the community's overall goals. Better racial representation could be included in those goals. So admitting more blacks isn't any more unfair than admitting more people with high IQ's.