@SCR0071
It isn't the male that solely determines the sex of a child- XX is a girl and XY for a boy - the chromosomes need to combine and that assignment is one last to be made.
Yes, rare DSDs aside, the male parent does determine the sex of the child.
Normally, each cell in the body apart from reproductive cells (gametes - sperm and eggs) has 23 pairs of chromosomes (46 chromosomes in total). 22 pairs are autosomes (ordinary chromosomes) and 1 pair is sex chromosomes (either XX or XY).
"Ordinary" cell division by non-reproductive cells by a process called mitosis produces new cells which are identical genetic copies of the "parent" cell, containing the same 46 chromosomes (23 pairs).
Gametes are produced by a different type of cell division called meiosis. As part of this process, the 23 chromosome pairs split up so that one chromosome from each pair moves to opposite ends of the cell (think of two lines of dancers facing each other in pairs, then the lines separating and backing away to opposite ends of the room). The cell then splits in half down the middle to make 2 new cells, with each new cell containing one chromosome from each of the original 23 pairs - so 23 chromosomes in total (half a genome) in each gamete.
At fertilisation, the sperm enters the egg and "half-genome" from the sperm combines with the "half-genome" from the egg to create a full set of 23 pairs of chromosomes in the fertilised cell.
In female gametes (eggs), the single sex chromosome in each gamete is always X, because the sex chromosome pair in a fertile female is XX, so X is the only option available. During male gamete formation, the XY chromosome pair will split so that half the gametes produced contain an X chromosome and half contain a Y chromosome. When the genetic material from sperm and egg combine at fertilisation, what determines whether the resulting embryo is male or female is which of the 2 sex chromosomes the fertilising sperm contains. An X-containing sperm will combine with an X-containing egg to produce an XX sex chromosome pair and the embryo will be female. A Y-containing sperm will combine with an X-containing egg to produce an XY chromosome pair and the embryo will be male. (Inevitable caveat: sometimes defects in individual genes necessary for reproductive development result in DSDs, so that reproductive system phenotype isn't typical of the sex chromosome pair present.)
Sometimes the process of chromosome pair separation during meiosis goes wrong, and instead of separating and moving to opposite ends of the cell, a chromosome pair will stick together and move to one end of the cell without separating. This means that when the cell divides in two, one of the new cells will have an extra chromosome (i.e. both chromosomes in a pair rather than just one of them) and the other new cell will be lacking a chromosome from that pair. When this happens to sex chromosomes, it results in specific DSDs such as Turner's syndrome (XO - one chromosome missing from the sex chromosome pair) and Kinefelter syndrome (XXY - an extra X chromosome).