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.
