Your doctor is pretty spot on. Practically any STD is a 1 in 10,000 sort of shot. Herpes heads the list of possibilities I agree, but also a very long shot and would be backed up by symptoms including lesions on your penis (but not if you already are oral HSV1 cold sores).
May well be a general UTI. Have little doubt you will be all clear of anything serious and will knock of this irritation pretty soon.