When it comes to sex, it's ok to discriminate on the basis of sex, because when it comes to sex the different sexes are very different. Their sexual characteristics are different, they produce different sex hormones, and they even participate in the act of sex in completely different ways.
Were there to be some activity where your melanin levels were critical (hard to think of a realistic one, but say you were going to be sent on a space journey where extra melanin would be a grave disadvantage nutritionally) then race (or really, skin color - I can't even think of a ridiculous example for race) would be extremely relevant. In that instance, discrimination based on skin color would make perfect sense whereas discrimination based on sex would not have any such obvious connection.
Were there to be some activity where your melanin levels were critical (hard to think of a realistic one, but say you were going to be sent on a space journey where extra melanin would be a grave disadvantage nutritionally) then race (or really, skin color - I can't even think of a ridiculous example for race) would be extremely relevant. In that instance, discrimination based on skin color would make perfect sense whereas discrimination based on sex would not have any such obvious connection.
Does it make sense now?