An Overview of Male Fertility: "There are a number of possible issues that can affect the possibility of a man fathering his own children.
Firstly there are the most obvious, but normally necessary prerequisites of achieving an erection and then being able to ejaculate.
The failure to achieve an erection is called erectile dysfunction – ED or impotence. This can be caused by stress or other psychological disorders and is relatively common. It can also be caused in older men by atherosclerosis, where blood flow to the penis is inadequate. Viagra is used by many to counter this problem, although like many drugs this can have side effects, the most common being headache."
