Sex cells do not divide properly, and there are too many or too few chromosomes in the egg or sperm. (discovermagazine.com)
From the time of puberty on, men make sex cells (in the form of sperm cells) continuously. (health.howstuffworks.com)
«Sexual reproduction does not require males and females, all it requires is the fusion of sex cells from two different mating types.» (sciencedaily.com)