July 16, 2008  -- It's all over the news -- couples breaking up because someone cheated. What is going on?

"Sex is the most primal urge in every single one of us," said advice columnist Steve Santagati. He makes money telling people things like that. He proudly markets himself as a "bad boy."

"A bad boy is a guy who's unapologetically male, loves being naughty, and loves seducing women," Santagati said.

He's all over TV these days. On "Oprah" and the "Today" show helping women understand men. On CNN, he offered this advice on the sex scandal that brought down former New York Gov. Eliot Spitzer.

"These guys are not sorry for what they did, they're sorry for only one thing, and that is getting caught," Santagati said.

'Bad Boys' Who Cheat

Santagati is 44 years old, single and said he never wants to get married or have kids. He believes men cheat for a number of reasons, but one may be that they settle down before they are ready. Santagati warns his clients that "you can't be in a monogamous relationship if you feel like you're settling or you're missing out."

His book, "The Manual: A True Bad Boy Explains How Men Think, Date, and Mate -- and What Women Can Do to Come Out on Top," is a best-seller.

Santagati is a former model who starred in a hit music video with Celine Dion. These days he runs a Web site, askstevesantagati.com where people can order his bad boy T-shirts, which are popular with celebrities like Lauren Hutton and Tommy Lee.

And on his Web site people, well women mostly, pay money to get his personal advice. He promises to be "brutal" with his advice -- to help women understand how a bad boy really thinks. This is helpful because women just don't get men, he said. But bad boys, he believes, do better with women because they can admit they have a wandering eye. And he said women like men who are bad boys.

"Everything inside of us, all of the blood running through our veins, is going 'look, look, look.' We cannot look at a woman's chest. We have to physically try not to," Santagati said.

He said many men get married too young and don't get to fulfill their "check list" of fantasies.

"We want the Swedish girl, the Hispanic girl, the black girl, the redhead girl, the brainiac," Santagati said.

He has a point. A study in the Journal of Couple & Relationships Therapy reported that half of married men and women cheat. A more recent national opinion survey from the University of Chicago said it's closer to 18 percent. But whatever the number, there's plenty of cheating and men cheat more. Even America's most desirable women are cheated on.

Why Powerful Men Cheat: 'Because They Can,'

There's always celebrity gossip about cheating spouses. Most recently, rumors were confirmed when Peter Cook admitted to cheating on Christie Brinkley.

"Jude Law fools around with his nanny," Santagati said. "And then I look at [Law's former fiancé] Sienna Miller and I look at the nanny, and I'm like, 'How did that happen?' I know how it happened. It was a ... fantasy. It was fooling around with the nanny. It was the, one of the naughtiest things he can do."

He said bad boys like being naughty; it inspires them to cheat. But why? Because "we're programmed for sexual variety," Santagati said. In other words, we shouldn't pretend to be shocked when even our so-called leaders are caught, he said.

Men in power cheat more, he said, because they can. Men like Prince Charles and Bill Clinton.

"Bill Clinton fooled around with Monica Lewinsky," Santagati said. "Now, no offense to Monica, but she, she's no Jessica Alba. She is ... the intern. She's the last person in the world Bill Clinton should have any sexual contact with. And that makes it extremely provocative."

Is Monogamy Just a Myth?

But it's not just people who cheat. Dr. Judith Eve Lipton, a psychiatrist, said that a common myth is that many animals are monogamous. She and her husband, David Barash, a zoologist, who co-wrote the book "The Myth of Monogamy," say there are a lot of misconceptions about monogamy.

For instance, many people grow up believing black swans, wolves and elephants are happy, monogamous mates. But scientists now know that "virtually no animals practice sexual exclusivity. They keep house together year after year in many cases, but they're sexually promiscuous," Lipton said.

Barash notes that the film "March of the Penguins" was touted as a great peon to lifelong monogamy, but "the truth is, these animals ... remain monogamous, faithful to one partner, for one breeding season. The next breeding season they will choose a different partner."

Scientists used to believe many bird species were monogamous, but recently, they've found otherwise.

"The female birds go off in the bushes and have sex with somebody other than the guy who's sitting on the nest," Lipton explained.

Scientists around the world have tested the DNA of baby birds and found even those in the same nest had different fathers. All that is evidence, Barash believes, that monogamy does not come naturally.

"When it comes to human beings, there's absolutely no question about monogamy being natural. It's not," Barash said. Barash and Lipton believe it all goes back to evolution: The male's goal is to make sure his genes live on and therefore he sets out to fertilize as many females as possible.

"Sexual opportunity is the name of the game for males," Lipton said. Women, on the other hand, spend nine months pregnant, then have to care for their children. So it's in the interest of the woman to find one man who will stay with her, or at least help her take care of her offspring, and some might argue that man is preferably wealthy or powerful.

"Females, by nature, are more choosy and less opportunistic," Lipton said.

But of course women cheat, too, Santagati reminds us.

"Men cheat because we are ... programmed to cheat. But who are we cheating with? We're cheating with women. I've cheated before. I've been a cheater. I know that these women are in on it. Women have been cheating on their boyfriends to be with me at times," Santagati said.

Is Marriage a Mistake?

But according to Barash, we shouldn't lose all hope in monogamy. There are a few animal species that are sexually faithful. Like the Malagasy giant jumping rat, the fat-tailed lemur and the California mouse. "You have to dig pretty deep to find other species that are … truly and reliably monogamous," he said.

"We're less like a Malagasy giant jumping rat than we are like ... chimps and ... gorillas and our primate ancestors who are not monogamous," Lipton said.

With so much cheating going on, Santagati tells people that they should "not go and think that you need to get married, have two kids, and live happily ever after. That, my friend, is a load of crap for 90 percent of the population."

So are most men who get married making a mistake?

"I would venture to say they haven't thought it through," is Satagati's confident response. But why should anybody take his advice?

"I have spent every waking breath since I reached puberty to understand women," he said. "The more I learned about the opposite sex, the more I knew how to get girls."

But maybe women just like him because he's good-looking and charming? Santagati insists that's not the case.

"You will see beautiful women with unattractive guys all the time. Women want to be with real men," and according to Santagati, real men cheat.

The Merits of Matrimony

Brown University professor and psychiatrist Dr. Scott Haltzman said Santagati's message is nonsense. Haltzman is the author of the book "The Secrets of Happily Married Men".

"There's no arguing that having an affair is full of fireworks and excitement and incredible energy. But that fades. That always fades," Haltzman said. "The type of quality you get in a relationship with your wife is something over a period of time that cannot be replaced by any affair or any fling."

And many studies do show that married couples on average, are happier. This year, Brigham Young University professor Julianne Holt-Lunstad found married couples are healthier, too. They have lower blood pressure, less stress.

Some still argue that monogamy isn't natural and that men, especially, are biologically programmed to stray.

"Well, we're programmed to do a lot of things," Haltzman said. "It may be natural if I'm mad at my boss, to want to punch him right in the face. But just because that's a natural thing to do doesn't mean that I'm going to do it."

And although Barash and Lipton concluded that monogamy among animals and people is not natural, they have been married, and faithful to each other for 31 years.

"It has been largely wonderful," Barash said. "The myth would be to say it's always been wonderful. But it certainly hasn't. It's been largely wonderful."

It's been wonderful for Haltzman and his wife as well.

"The benefit of being with one person is that you know that when you're making love to that person, what you have with that one individual is something that you have with no other person," Haltzman said.