Normal mouth bacteria can cause infections in the urethra, as can the bacteria that
causes strep throat, adenovirus (usually
causes upper respiratory viruses, bronchitis, etc.), gardnerella (the bacteria that causes bacterial vaginosis), yeast, to name just a few.
You can also get it from irritants, like soaps, spermicides, or any product you might use in the area. It might also be a urinary tract infection or prostatitis.