Hematospermia, or blood in the semen, is commonly caused by some kind of infection — either of the urethra, the prostate or the seminal vesicles. Sometimes the hematospermia is triggered by prolonged physical activity such as bicycle riding. It is usually benign, a thorough history, but if you find blood in your semen a physical examination and laboratory testing should be conducted in order to rule out something serious.
Other common causes of hematospermia include:
Procedures to sample prostate tissue (prostate biopsy)
Calcium buildup inside the prostate (prostatic calculi)
Lesions or polyps of the urethra
Trauma (i.e. a blow or injury to the groin area)