This might be the highest risk from fellatio, sometimes perhaps due to normal oral bacteria and not a standard
STD. But nonchlamydial NGU is not a serious
STD, with no known complications or serious outcomes -- so really not a big deal.
Herpes: This is the last possibility and the only one that would not be prevented by azithromycin. Genital herpes due to HSV-1, the usual oral herpes strain, can happen but is rare after any single exposure.