I've notice a fishy odor after we have eaten seafood. Maybe that's an answer to Roaamaria526's question.
A "fishy odor" is most often described as a complaint from women with vaginal discharge and an infection called Bacterial Vaginosis (BV). BV is very common and esaisly diagnosed and treated by your OBGYN. As to why you might occasionally smell such an odor on you partners penis, I would have to defer those questions to hm.