Sometimes WBCs are found but very few. I have an appointment with a urologist......in January!!!
It could be from bacteria in the mouth if you received oral sex. It could be strep, even adenovirus, and others along those lines.
https://emedicine.medscape.com/article/438091-clinical#b5
"Commonly identified causes of NGU include the following:
Chlamydia trachomatis (15-55% of cases)
Ureaplasma urealyticum (40-60% of cases)
Mycoplasma genitalium (15-20% of cases)
Trichomonas vaginalis (< 5% of cases)
The number of fastidious organisms implicated in NGU is increasing and includes several Ureaplasma and Mycoplasma species. The causative organism cannot be identified in most patients with NGU.
Rare cases may be related to lymphogranuloma venereum, herpes simplex, syphilis, mycobacteria, or urinary tract infection with urethral stricture. Other rare but reported causes of NGU include anaerobes, adenovirus, cytomegalovirus, and streptococcus."
Some people never find out the cause.