| Hi Junior GP,
It is really difficult to say what could be causing this, though there are two other possibilities that come to mind off-hand. You may have what is known as an "occult" prostate infection. In such a case, the infection can be relatively walled off and confined to the prostate, making it pretty well undetectable in the urine -- though it should be detectable in semen. A manual prostate exam should easily confirm or rule such out.
Another more remote possibility is a yeast infection. This would not be measurable by a urine test nor treatable by antibiotics -- in fact they may well serve to worsen such, particularly with longer term administration.
NSU can be caused by a vast array of different organisms and the cause is not always sexually related. Often the initial cause can not be determined. If you were the penetrative partner, unprotected anal intercourse could very likely be the instigating factor. Though, if the receptive partner, it would likely be far more difficult to determine if this is in fact the indirect source of your NSU. Either way, please use protection in the future as you otherwise expose yourself to fairly great risks either way.
Take care. | 
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