What causes an erect penis not to be hard enough to penetrate?

It depends on the individual. If the male is middle-age or older, it could be caused by restricted blood flow through the Dorsal Artery or any point of blood flow to the Dorsal artery. Blood flow restrictions can be caused by arterial plaque or other physical causes. Diabetes can cause similar erectile dysfunction due to lowered testosterone levels. A man's testosterone level declines about 10% per decade. Symptoms of a lower testosterone level can include a lower libido, impaired sexual function, less energy and drive, and body changes such as reduced muscle mass and physical strength as well as increased body fat, especially around the waist. The incidence of erectile dysfunction that increases with age is partially due to decreasing testosterone level. It can also be related to blood flow restrictions caused by disease processes. For example erectile dysfunction is much higher among diabetics than among non-diabetics.