The cackling can sound like a chicken who is facing his certain death; the eye rolling suggests that vaudeville is not dead.

Whenever unlikely lovers pair up, onlookers are rarely without an opinion or the kinds of noises you would expect from third graders shown videos of childbirth. It doesn’t matter if the couples’ unlikeliness is based in their disparate ages, their levels of attractiveness, status, physical size or race: Show them a Rupert Murdoch or a Madonna, and the world goes all Joan Rivers.

We’ve grown inured to many of these responses. It seemed only inevitable that the British gossip website Popbitch would invoke the phrase “Gruesome Twosome” to the recent marriage of Rupert Murdoch, 84, to Jerry Hall, 59, and we saw an equally measured response to US magazine’s reporting on Thursday that Mr. Murdoch’s former wife Wendi Deng Murdoch, 47, is dating Vladimir V. Putin, 63, the president of Russia.

Business seemed as usual when an online fan of the TV show “Empire” responded to last fall’s love scene between the plus-size actress Gabourey Sidibe and a gym-fit actor by posting online, “Well this is definitely in the Nat Geo category of non-arousal.” It’s no surprise that when the French banker Olivier Sarkozy started dating Mary-Kate Olsen — the designer and former actress, who is now his wife, and is 17 years younger and 13 inches shorter than him — Mr. Sarkozy’s former wife, Charlotte Sarkozy, would say: “That’s not right. It’s grotesque.”

But it’s slightly bizarre when your otherwise hip and socially liberal friends make the same kind of comments, particularly when the couples in question are not on the child-producing or child-rearing path. What’s going on here? How is this any different from racism or homophobia?

“It’s absolutely not different in almost all ways but one: It is tolerated in our society,” said Ben Michaelis, a clinical psychologist. “Unfortunately when people don’t understand certain other people, they tend to place them as an out group, psychologically. Then one of the ways they bring their psyches back into balance is to lower these out groups, and the other way is to attack them.”

Celebrity couples are, of course, particularly susceptible to this kind of scrutiny and biliousness. Robin Dunbar, an anthropologist at Oxford University, said: “Celebrity watching has to do with making sure the famous are not breaking the rules too much. We pay for them to be up there.”

For some fans it’s even more personal. Elaine Lui, a Canadian gossip maven, a reporter for CTV’s “etalk” and a host of the talk show “The Social,” said: “When a celebrity gets together with someone who fans perceive is not on his level, it’s almost an insult. You’re like: ‘I cast you in this glorious light. If you pick someone who is close to me in status or looks, then why did I put you on a pedestal?’” Thus the tabloids’ fascination with lithe male actors and their zaftig, or conventionally shaped, wives.

But when the couple is not famous, the motive behind the cattiness is less clear. Dr. Dunbar has written that gossip is often a form of social order, a way to cement bonds with your own tribe and to assert values with them. He said, “In most small-scale, traditional, hunter-gatherer societies, typically the word for your tribe is simply the word meaning human, and anyone else belongs to a category that includes all the other animals.” To joke with a friend about a millionaire chief executive dating an impecunious graduate student is to acknowledge that the society we both live in is one in which magazine articles titled “Who Makes What” are not considered vulgar.

Indeed, the inappropriate comments aren’t necessarily derived from intolerance. Victoria Binda, a teacher in New York City who is a white woman married to a black man, said that when she and her husband introduce themselves to new acquaintances, “almost everyone asks if we plan to make beautiful mixed babies, since mixed babies are, as apparently decreed by some higher authority on infantile beauty, the most beautiful. My husband and I do not plan on having children, a decision that inevitably disappoints our audience and all of the human race. Some have suggested we are refusing to contribute to a better and more blended society.”

But that’s not the only thing on people’s mind when they meet Ms. Binda and her husband. “Many other women feel it’s acceptable, even clever, to comment on our sexual relationship,” she said. “They assume that my husband is well endowed and that we engage in a lot of wild extracurricular activities.”

David Gilmore, a photographer and graphic designer, recounted riding on Amtrak in California in 2010 with his boyfriend, who was 21 years Mr. Gilmore’s junior. Mr. Gilmore said: “A conductor recognized me from a previous trip and said: ‘Hey, welcome back. Is this your boy?’ To which I shot him a stern look, and he got it. He said: ‘Oh, he’s your boy! Got it.’” Mr. Gilmore said of the incident and his partner: “I think it hurt our relationship. He already had his mother being homophobic and ageist.”

Certain anxieties seem more justified than others. To be sure, uncharitable comments leveled at older men who repeatedly throw over their wives for increasingly age-inappropriate mates are of a different order from other kinds of deprecations, given the lack of originality on these conquistador-husbands’ part.

Ditto the instances in which one partner is very young: that Eugene O’Neill disinherited his daughter, Oona, after the 18-year-old married the 54-year-old Charlie Chaplin is understandable in the same way that the public’s bafflement about Woody Allen taking up with Soon-Yi Previn was. Even Mr. Gilmore, who is in a relationship with a man 23 years his junior, has sympathy for the woman who’s been dumped by her husband for a trophy wife. “My first thought is: ‘Poor woman. How could you not see it coming?’” he said. “Then, ‘Hope you have a good divorce lawyer.’”

Snarky comments made about other unlikely alliances, however, often bespeak a callousness or a lack of esprit. When you marvel or jeer at the fact that one of the stars of TV’s “Nashville,” the petite actress Hayden Panettiere, is with the hulking boxer Wladimir Klitschko — a man 16 inches taller, 14 years older and unknown tons heavier than she is — are you unintentionally asserting the values of your culture, or are you broadcasting to the world the impoverished dimensions of your imagination?

The world’s oldest story rarely fails to rattle. Dr. Dunbar, the Oxford anthropologist said: “I suppose the question is, ‘Which is more surprising, Rupert Murdoch marrying Jerry Hall, or if Jerry Hall married a 20-year-old?’ One look at Rupert Murdoch, you shake your head and think, ‘What on earth is Jerry Hall thinking?’ But if it were the other way around, you’d be absolutely astounded.”

Many would. But to be so would be to make the world a smaller and meaner place. A place where the paradigm of romance and love is the crowded and smelly Noah’s Ark. A place where ultimately the joke may be on you. “I’ll look at a couple walking down the street and think, ‘He’s on his third marriage and she’s a gold digger,’” Ms. Lui said. “But even if that were true, she’s getting hers and he’s getting his. Sure, I’m being mean about it, but I’m the only person being hurt by it because I’m not getting mine.”