What determines if baby is boy or girl?

The lucky swimmer. The egg can go either way. The sperm carry either a male sex factor of a female sex factor. The sperm that gets there first and fertilizes the egg determines the sex of the baby. There are no special diets or sexual positions that change that.It is all up to the sex factor brought by the sperm that gets there first.
Random. The egg has an x-chromosome. Sperm have roughly 1/2 x and 1/2 y. The winner of the race to fertilize the egg will determine the sex. Females are xx and males are xy.