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A male cyclist can place a significant percentage of his weight on his perineum, an area between the scrotum and the anus where the nerves and arteries to the penis pass. This pressure - and a narrow saddle seat - can injure the arteries and nerves.
The earliest signs indicative of damage include numbness, pain and decreased erectile function.
In case, a trial of medications of Cialis or Viagra are helpful, then psychologic causes could be the main cause of your symptoms.
The best way to make sure, there is nothing wrong with blood flow in the penis, would be with a color doppler sonography (with prostaglandin injections).
This test will help to grade the level of erections, achieved, and help to rule out vascular insufficiency as cause of your issues. In such a situation proper surgical intervention (venous ligation) might be the solution.
You might consult with your doctor or a urologist for additional evaluation and proper advice.