In season one, "Fry and the Slurm Factory", Bender uses the F-ray on Fry.
BENDER What should we point it at first? FRY I 'unno. Try it on me. Ow! My sperm! BENDER Wow, neat! Mind if I try that again? [He points it at Fry's crotch again but nothing happens.] FRY Huh, didn't hurt that time.
This ray is so strong when the professor uses it he is in a gigantic radiation suit. Then in season 3 "Roswell That Ends Well", and continuing forward it is stated that Fry is his own grandfather. How can this be if his "bits" were irradiated?