Milky discharge during defecation occasionally, not associated with any other symptoms, what can be the cause?

Probably not serious: Presumably you mean discharge from the penis. This symptom is common in many men, some more than others. Straining at stool or stool passing over the prostate gland and seminial vesicles (the sacs that store semen for the next ejaculation) can squeeze out semen or prostate fluids. If this is new or if the fluid is creamy (like pus), see a doctor to be sure no prostatitis or other infection.