Why Do I Have No Semen Discharge From Penis ?

I am 33 years of age and married for the last 7 years. I have two kids, son and daughter each. During erection, drops of colorless fluid discharge from my penis. otherwise no semen discharge occurs while urinating in the day or night or in any other form. only this problem occurs and soon after when I urinate it clears that fluid. This problem is so old that I can't even remember that time. you can say from a decade or so. I have taken medicines couple of times but it didn't work expect decreasing the amount of that fluid than I stop using medicines because one of the very experience and famous hakeem sb of our city told me that this is natural and its purpose is to provide lubrication. but now again some one told me that this is a disease. I have read about you on internet and therefore request you to please clear this confusion in my mind and also prescribe some medicine. I will be great full to you.

Regards.

Murad