Suppose that you are thinking of having a child:
......Your child will be like you, to some extent, and will to the same extent be like his or her father, just as half of your genes come from your father and half from your mother. So, your child's genes will be 50% your man's, 25% your father's, and 25% your mother's. And of course, what goes for you goes for your mother, and for hers, etc. So:
......Your child's genes will be 50% your man's, 25% your father's, 12.5% your maternal grandfather's, 6.25% your maternal grandmother's father's, 3.125% your maternal grandmother's maternal grandfather's, etc.; i.e. they will be 50% male genes + 25% male genes + 12.5% male genes + 6.25% male genes + 3.125% male genes + ... = 100% male genes.
......Everyone has a biological father and mother, and so we have a mathematical and, to some extent, empirical (and of course fallacious) argument that all men are men.