The reason pornography sells well and strip dancers are so enjoyed by men is because it often elicits this response of physiological well-being. In gay men, the arousal mechanism works the same, but the object of sexual attraction is another man. Erection occurs after a period or arousal. In young men it may be often very soon after initial arousal and without any need for manual stimulation of the penis. As men age, arousal can continue indefinitely without erection which may need additional manual help.
Erection occurs when the spongy tissues of the penis are engorged with blood.
There is one large primary artery bringing blood into the penis, and several smaller veins that drain blood out of the penis. When not sexually stimulated there is a balance of blood flow in and out and the penis is flaccid. However, when stimulated, the artery becomes wide open, the heart rate and blood pressure increase and more blood is pumped into the penis. At the same time, valves in the veins controlling the outflow are squeezed shut so the penis enlarges. As men age, sludge builds up in the artery and blood inflow slows.