All my knowledge of this is second-hand, but here it is as I understand it:

Spider-Man: Reign (the comic that plotline comes from) is a story set on some other Earth in the Marvel Universe that was basically designed to be Spider-Man's equivalent to The Dark Knight Returns. In an effort to make it edgy and give Spider-Man a reason to be angsty, the writer had it that Spider-Man's sperm (and for some reason specifically his sperm) was radioactive as a result of the spider bite which gave him his powers.