Welcome,
First of you need to run 2.45.03Beta, that is the only modern version of the program where sexual reproduction works.
Here is the sexrepro gene from one of the robots I used to test sexrepro:
cond
*.robage 100 =
start
50 .sexrepro store
stop
cond
*.robage 50 >
start
-8 .shoot store
stop
The robot will start firing sperm at age 50, and attempt to reproduce sexually at age 100.