it requires that each parent has its own unique genetic distribution, so there is a possibility of one, or the other, in a genetic makeup.
This means that there would not be any "variation" if you mated a male and female clonal equivalent (although how we would have that is an interesting question). So, ultimately, somewhat as with the chicken and egg dilemma, you have to, at some point, have genetic variation arise of its own. Once it exists, then it is just random draws of colored balls from a bag. The colors that you get can vary, just randomly. Half from mom, half, from dad.