What an Idiot: Thom Holbrook wrote an essay pointing out that Superman is dumber than a bag of hammers in Returns.
The big inspiration for Superman Returns is when Superman and Lois had sex, what if he got her pregnant? Thus the need for him to be gone five years so that he could return to discover he has a son. That also means that, clearly, Superman left before he knew Lois was pregnant, we can assume he left shortly after Superman II. Really shortly. He's Superman. He would be able to see Lois was pregnant very early on with his vision. Lets say he leaves… two months after the end of Superman II. So in Superman II he painfully learns that he must put the needs of Earth ahead of his own personal desires. He apologizes to the President for being away so long (we was gone maybe a week or two) and that he won't let him down again. Then two months later he leaves Earth without telling anyone on a personal five year long wild goose chase. He also leaves without covering his bases. He doesn't secure the dangerous tech in his Fortress of Solitude. He doesn't make sure his departure won't result in Lex Luthor's release from jail. He doesn't even apparently warn the President he's leaving. His actions fly in the face of everything he learned in Superman II and everything he swore to do. Dumb as a bag of hammers.