Why does the male parent's genes determine the sex of the baby?

The mother has two 'X' chromosomes so all the mother has to offer to the new born are 'X' chromosomes the father has and 'X' chromosome and a 'Y' chromosome so a father either contributes an 'X' chromosome or a 'Y' chromosome. That is why the father determines the sex of a child.

Consider this:

Each parent has two sex chromosomes.

The mother, as a female, has chromosomes XX.

The father, a male, has XY.

The mother will pass on one sex chromosome to the baby, as will the father.

The mother can only pass on an X.

The father, however, has a 50/50 chance of passing on an X or Y.

XX would make a female and XY would make a male.

Simply put, the sex chromosome from your mother is fixed, it's going to be an X no matter what. It's which sex chromosome your father passes on that determines your sex.