"Mike must learn human customs. He must take off his shoes in a mosque, wear his hat in a synagogue, and cover his nakedness when taboo requires, or our shamans will burn him for deviationism. But, child, by the myriad aspects of Ahriman, don't brainwash him. Make sure he is cynical about it" (12.27)
Every society has its customs. We leave apples on teachers' desks, brides wear white on their wedding day, and we watch new Steven Spielberg movies even though we know they'll be awful (except Super 8—we're just sayin'). But we seldom question these customs because they're just so cozy and easy to roll with. Jubal isn't saying customs are bad; in fact, they're necessary. He is just saying that we should question them and not accept them simply for how easy they are.
"Okay, so I'm from Kansas. Never was any cannibalism in Kansas" (13.4)
Duke confuses Kansas's stance on cannibalism as a universal view on the issue. Depending on where you're born, your customs will be very different. East Coast and West Coast. The Northwest and the Bible Belt. California and Maine. All these areas are part of the United States and everybody who lives in them are Americans, but the customs between them are majorly different.
"It is almost impossible to shake off one's earliest training. Duke, can you get it through your skull that if you had been brought up by Martians, you would have the same attitude toward eating and being eaten that Mike has?" (13.83)
When you're raised to believe certain things, it can be hard to see them as not being true and even harder to shake free of them. Remember that sad day you learned about Santa Claus? (We do.) But the silver lining for someone like Jubal is that it's almost impossible, not plain impossible.
"Mike is utterly civilized, Martian style." (13.101)
Jubal uses civilized here as a relative term defined by culture. It all depends on the definition you give to civilized. Given the right definition and standards, do you think Jubal would think the cast of Jersey Shore is civilized? Maybe? Eh?
"Jubal, if I offered Mike a glass of water, would he go through that lodge routine?" (17.281)
Later in the novel, Duke has a whole bunch of water brothers. Just goes to show you how every custom and tradition has to have a start and an end somewhere.
"But a poor portrayal is as effective as a good one for most people. They don't see defects; they see a symbol which inspires their deepest emotions; it recalls to them the Agony and Sacrifice of God."
"Jubal, I thought you weren't a Christian?"
"Does that make me blind to human emotion? The crummiest plaster crucifix can evoke emotions in the human heart so strong that many have died for them. The artistry with which a symbol is wrought is irrelevant." (30.53-54)
Customs and traditions can give everyday items a huge value, particularly emotional value. This value extends to everything from religious paraphernalia to heirlooms to pop culture. We like 'N Sync and The Power Rangers not because they were good (they weren't), but because we invested so much emotion into them as a kid. The mind is deliciously tricky like that.
Patricia Paiwonski gave Ben Caxton the all-out kiss of brotherhood before he knew what hit him (31.1).
Basically, Ben makes the mistake of entering a room and thinking the customs on the other side of the door would be the same as his. Do this, and you might be in for a shock. In this case, a good shock, but a shock nonetheless.
He's still grokking clothes. He groks they're a wrongness that keeps people apart—gets in the way of letting love cause them to grow closer." (31.201)
Mike sees clothes as totally arbitrary—how we buy clothes, where we buy clothes, when we should and shouldn't wear them. What body parts the clothes need to cover up is equally arbitrary (see a 1920s male swimsuit for a perfect example). Mike just takes it one step further.
"All your stomach can reflect is prejudice trained into you before you acquired reason." (33.82)
Sometimes our minds are so ingrained with a certain custom or tradition that our bodies follow suit. Since Ben's mind controls his body, he feels physically ill at the prospect of something so foreign to his traditional understanding.
"You are married. After tonight there will never be any doubt in your mind […] I love them—all my brothers, both sexes." (32.52)
Duke introduces a new take on the tradition of marriage. The between-a-man-and-a-woman definition is tossed in favor of a more communal one. But, as we see in the "Language and Communication" section, this definition can be customary in Mike's community and still not so in Ben's, even though it's the same word.