Bacterial Vaginitis

I was recently diagnosed with BV, and was prescribed and treated with, 'Clindesse 1 Complete Course', a only one-time dose of Clindamycin vaginal cream. I was also told by my obgyn that I should avoid vaginal intercource  because of the different pH levels that I have vaginally and my boyfriends semen. She also added that most likely my BV was from my boyfriends semen. Can anyone explain to me why my vaginal discharge smells so fishy? I know that it is a common symptom of having BV, but I'm just curious if anyone knows why?