Ejaculation of semen is entirely involuntary in the human male, and in all other forms of terrestrial life. It would be unreasonable to assume otherwise for a kryptonian. But with kryptonian muscles behind it, Kal-El's semen would emerge with the muzzle velocity of a machine gun bullet. (One can imagine that the Kent home in Smallville was riddled with holes during Superboy's puberty. And why did Lana Lang never notice that?)