The sperm shot sends a copy of the robots dna over to the robot that is hit. The hit rob can then decide to reproduce sexually using this dna.
The dna only stays for a limited time indicated by *.fertilized (10 cycles).
These shots act like the other shots found in the sim. There is not much need for simulating tiny sperm cells. Also, I don't think single-celled organisms use sperm anyways.
If you wanted, you could make a multibot that released sperm "cells" to go shoot sperm shots. That would be fun to figure out.
http://wiki.darwinbots.com/w/Sysvar is your friend.