Fresh sperm has very little smell to it. It has a sort of flour and water smell. (It reminds me of the flour and water paste mix we used to use at junior school)
If yours smells bad when it is fresh, you may want to get checked out by a doctor.
Because particles are released from the sperm as it evaporates. Same reason why anything has a smell.Why do I smell so bad down there?
that's like asking why is the sky blue?Why does my period smell so strong?