You're close Lixy...it's a chromosome thing. Women have two X chromosomes in every cell, men have an X and a Y. Eggs have one chromosome from the mother (has to be an X, it's all she's got). Sperm have one from the father (either an X or a Y). If an X bearing sperm gets to the egg, it adds up to XX = girl baby. If a Y bearing sperm get there....XY = boy baby.

But as everyone has at least one X, I don't think it's entirely right to say that inherently female. It is true that the sex is entirely determined by the father's contribution.

And now the biologist is gonna go perv . Sorry for the lecture, sometimes I just can't help myself.