It is very hard to tell what the liquid substance is without seeing it. According to the data you provided, the liquid substance probably is not discharge. The discharge is usually sticky, with changed color (yellowish), and is associated only with urinating (although it appears most often when urinating). Since discharge is usually a sign of infection, other symptoms like itching, burning when urinating, and painful urination would be expected as well.
The liquid substance you describe might be a product of some of the sexual glands (prostate, seminal vesicles, bulbourethral glands) whose secretion is a regular part of the ejaculate.
You might want to visit an urologist for physical exam. He might be able to recognize if the secretion is pathological and needs treatment.