Any history of unprotected intercourse with multiple partners ?
Is there any associated burning sensation ?
Any history of STD ?
What is your age ?
Since how long are you having this discharge ?
Any other associated sympXXXXX XXXXXke lower abdomen pain etc ?
Colour of the discharge ?
Other then STD other possible causes of penile discharge include balanitis, phimosis etc .In the absence of any of these other possibility is normal secretion for prostate or inflammation /infection of prostate can also result in penile discharge . If the symptoms persists a Urological consultation is required in this case .
With STD you can have severe burning every time on passing urine and discharge may be of more frequency .
The discharge can vary in amount from scanty to profuse, and in colour from clear to yellow/green. The timing can vary from loss in the morning only, to throughout the day. Source
I would like to add if the symptoms persists then a Urologist may be consulted .
Also since your test for common STD ( gonorrhoea and chlamydia ) have already come back normal and not a usual presentation of STD , Also no history of multiple sexual partners .
This can also be normal secretions from seminal fluid secreting glands especially since there is no abnormal symptoms like severe burning, pain , penile lesions etc .At the most you can also go for urine culture and sensitivity for further evaluation .