Nope, there are dozens of things that can cause NGU.
NGU means non-gonoccocal urethritis, so it's basically any infection in the urethra that isn't gonorrhea. Often, it's an STD, but it isn't always.
Does your wife perform oral sex on you? 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.
You didn't have it for years and not know it, but there are all kinds of things that could have caused you to get it recently other than her cheating. This alone is not nearly enough evidence to prove that she cheated.