You have Semen Leakage ( Spermatorrhea). Semen production goes on in the testis round the clock and gets stored in seminal vesicles, Prostate and testicles. Now imagine these organs as water tanks. Continuous production at some time, will make tanks full. If the boy or man is not married, or not sexually active or not mastrubatiing, their storage capacity runs out. Now overflowing mechanisms take place like night falls and seminal discharge in urine. It’s perfectly normal. Nothing to worry about it. To avoid this one needs to have sex or masturbate regularly. It won’t lead to any problems.