It is possible that his prostatitis is from an STD you passed to him from your extramarital affairs. I would get tested for all STDs and refrain from going outside of your marriage.
You can get prostatitus from bacteria and non bacteria type sources. It is kind of surprising that his doctor did not run at least a urine analysis and check for infection. The minimum the doctor could have done is check his prostate and a digital rectal exam which might have been enough to prescribe antibiotics. Not all prostatitus is related to bacteria infections..
If it is bothering you enough, then think about getting tested for bacteria type stds... You typically get results from your PCP in two to three days...