Micdavis beat me to it, but I'll repeat his sage advice: Read Larry Niven's very funny: MAN OF STEEL, WOMAN OF KLEENEX. He talks about how all living Kryptonians have super powers under our yellow sun... so, technically, Kryptonian sperm being 'alive' they'd have super speed and invulnerability and the sperm would "gut Loise Lane from crotch to sternum, lastly blowing the top of her head off" Then there's the problem of super sperm shooting through the skies of Metropolis at super sonic speeds... and if the adolescent Kal-El plays with himself we have the same problem all over Smallville...
It's a straight-faced examination of the problems of a super human mating with a mortal.
Thanks, Mike!