The list of causes of blood in the urine (called hematuria) is long and varied. Does this mean he has an STD and has been cheating on me? is not discovered, and can develop anywhere among the kidneys, ureters, prostate and bladder. Aug 12, - Seeing blood in the semen can make a man anxious. of semen); Urethra (the tube that carries urine and semen from the penis) It can also come from an STI (sexually transmitted infection) such as gonorrhea or chlamydia.