Urine and semen can never mix together i.e. normally; one cannot pass semen in urine. Sometimes, a physiological alteration in the urinary solutes may change the appearance of urine, making it whitish, which is mistaken to be semen by misinformed individuals. In reality, it is a secretion of the prostate and the urethral glands. It is common in constipated individuals. Treat your constipation. Stop worrying and try to take your mind away from that.