I think in understanding the human condition of being in love it is consistent not say goodbye to Earth, but he would say goodbye to Lois, especially under the circumstances.
I think in understanding what a commitment is, it is consistent to say goodbye to both Earth AND Lois, especially under the circumstances. If Earth has some threat coming over, they should at the very least know they shouldnt wait for Superman to come. Watching Superman II we can see how desperate and devastated people of Earth were, as wondering Where is Superman? Why isnt he here to defend us from Zod?
How is consistent not to say good-bye to people of Earth that trust Superman will be there to protect them just because hes in love? Being in love inhabilitates him to at least communicate hes retiring?
THe relationship he has with Earth is different from the relationship indicated in SR. If he was not in a personal, sexually intimate relationship with Lois, I wouldn't expect him to say goodbye. HOwever, he is, and that makes the relationship different from his relationship to Earth.
A commitment is a commitment. If hes going to leave alone someone hes commited to, he should at the very least tell them. Not to mention that he should not quit his commitment to start with.
Do you see how they are different and would elicit different behaviors? THey are not the same.
The nature of both commitments might be different, but a commitment is a commitment. He cannot think that he owes to his romantic commitment a good-bye and to his commitment to people of Earth an absolute nothing.
In fact, he doesnt think that way. As soon as he defeats Zod he goes to the White House an offers an apology to the President for such a careless behaviour. Much like he did with Lois on SR.
He doesnt go to the President and say Well, I can quit whenever I want to.
Also, I don't think he should have to say goodbye to Earth as a whole in the context of Superman II, nor do I think it was a mistake for him not to say goodbye to Earth in Superman II. THere is no obligation on his part. He is not a public servant employed by the government. He is a private citizen acting on his own volition to help people. He can stop anytime he wants without an explanation.
He is invested in this mission of defending Earth by Jor-El in this franchise. Hes not doing it for the fun of it or to spend a boring afternoon.
He cannot stop anytime he wants and thats exactly what he learns in Superman II.
And that is why he goes back to the FoS saying he failed. Because he thought he could quit his mission and his commitment in order to have a normal life, but he realises that he cant. He realises that his commitment to people of Earth is in fact more powerful than his romantic commitment and he acts consistently, he drops Lois and embraces his mission once again.
However, not saying goodbye to Lois in the context of SR IS expected b/c he is in an intimate sexual relationship with her. This is a specific situation that obligates one to be responsible and communicative with one's partner. Especially if you really love that person. There is nothing to indicate that he would act otherwise.
There is. Again, the fact that another commitment that proved to be stronger than his commitment to Lois provoked him no desire to say good-bye. The same his commitment to his original race made him noit to say good-bye to Lois. He was sure that if he told good-bye to her, he would have ended up not going to Krypton because of his love feeling towards her.
Sex or not sex with the people youre commited to, a commitment forces you to at least communicate your quitting it if that situation happens.
As far as his mission placing Earth above Lois (or any one person for that matter), that is how you know he is human. Eventhough Jor-El WANTS him to place his mission to protect Earth above any one human, the fact that he IS emotionally human causes him to place Lois first. THat is simply the actions of a man in love. It is inconsistent for him not to suck it up and tell Lois goodbye and spare her feelings what he can instead of placing his feeling first.
He placing Lois over humanity is an action of a man in love.
And in Supermans case it is a mistake and the wrong thing to do and he ends up learning this in Superman II. Which proves that Superman makes mistakes, just like in SR and mistakes of the same nature. Except that in SR he didnt put Lois over his missions as Superman as he did on SII, but the other way around.
Now, placing Lois over humanity in NO case denies his obligation to communicate people of Earth he will be there no more to protect them. He might have to lose his powers but not the obligation to say Now you shouldnt wait for my help anymore. Anything that goes wrong, you are now by yourselves. Be warned about this. Good-bye. Superman. He is not incapacitated to say this if hes decided to quit his mission since people on Earth count on him and trust him immensely in case of danger. Sure, maybe people of Earth have no right to firce him to protect them, but at least he has to be thankful enough for all peoples love telling them hes retiring.