The dad says, "Boys, a little fatherly advice for you. Never cut a burger in half. You look like a pussy doing that. You hold the whole thing in your hands like a man."
Well, I can feild this one, but first let me get #NotAllMen out of the way. ^There's your answer. We get taught these things by men, as boys. I did, because I was misdiagnosed as male at birth, so I grew up in the locker room, and so forth. My father didn't teach me this Burger Rule, but I heard it.
That's how it gets around, you grow up around other boys, who pass on this Power Assertive (That's basically the set of behaviour we're talking about) advice from. It's distaff counterpart is "Smile, you're never going to get any attention from boys walking around with a sourpuss all the time. You know, you'd be so much prettier if you smiled more?"
Real men drive muscle cars, that's how you pick up chix, and for the love of God never cut your burger in half like a pussy. Swallow it hole, like a man. We even have a topic here, of
Advice for My Boys. It's almost as if boys don't have any more choice in what sexist stereotypical behaviors they're allowed to do than girls are.