Why is the sex of an organism usually determined at the time of fertilization, i.e. when sperm and ovum merge?

In determining the sex of the unborn child, the Y chromosome plays the main role. When an egg is fertilized with a sperm carrying the X chromosome, the female body will develop, it follows that the girl has 2 X chromosomes, one from the father, the second from the mother. If the sperm carries the Y chromosome, then the male body will develop, therefore the boy receives the X chromosome only from his mother.