Although I get the impression that the OP probably knows the answer and is just fishing for interesting replies, I'm going to rise to the bait by saying that I think the simplest biological explanation is best - xx/xy chromosomes.
Keeping it simple means that girls and women can be as wonderfully complex and varied as we are in appearance, opinions, abilities, ethnicities, capabilities, jobs, clothing, interests, sexualities, etc., and still be human females.
And boys and men, despite their appearance, opinions, abilities, ethnicities, capabilities, jobs, clothing, interests, sexualities, etc., will always be human males.
Works for me!