Yes sometimes a man's sperm can cause a woman's vagina to smell fishy and I've noticed that before when me and my husband had sex.
I thought something was wrong at first when I noticed a fishy odor in my vagina but I spoke with my doctor about it and the doctor said it was okay and normal and that the fishy smell should go away within a few days.
But if your vagina continues to smell like fish then you could have a Urinary Tract Infection which can be treated by a doctor.
Urinary Tract Infections are not always very serious but you should still have the urinary tract infection treated.
If you have a Urinary Tract Infection you can draw a warm bath and add 2 cups of Apple Cider Vinegar to the warm bath water.
The Apple Cider Vinegar can help kill off the bacteria causing the Urinary Tract Infection when you soak in the tub for 30 minutes to an hour.