President Obama just condemned an ancient double-standard. "We need to change the attitude that punishes women for their sexuality," he said Tuesday, "but gives men a pat on the back for theirs."

He declared this during the White House Summit on the United State of Women, a conference that tackled reproductive health, among other issues. The crowd of advocates, businesspeople and students exploded in applause.

Taking aim at a number of gender inequalities, the president called himself a feminist.

"I might be a little grayer than I was eight years ago, but this is what a feminist looks like," he said.

Obama might be the first sitting president to publicly denounce the perception that men are studs for racking up lots of sexual partners and that women who do the same are, well, terms we can't use here.

"We're going to have to change the way we see ourselves," he said. "This is happening already, but we need to be more intentional about it."

President Obama speaks at the White House Summit on the United State of Women on Tuesday. (Saul Loeb/AFP/Getty Images)

Then he quoted Shirley Chisholm, the first black woman elected to Congress: "The emotional, sexual and psychological stereotyping of females begins when the doctor says: It's a girl."

Juliet Williams, a gender studies professor at the University of California at Los Angeles, said the comment, dropped into a 20-minute speech about economic opportunity, was a flash of progress. Presidents haven't talked about sex-shaming, she said, because Americans until recently didn't talk about sex-shaming.

"It was an everyday expression of a longstanding, traditional gender ideology that tethered a woman's social value to her chastity," Williams said. "Things have changed."

Second Wave feminists, she said, sparked a seismic shift in how Americans hooked up. "Activists legitimised the idea that women not only have sexual desire but are entitled to sexual agency," Williams said.

Betty Friedan, author of 1963's "The Feminine Mystique," inspired both praise and outrage for suggesting women enjoyed sex. "No woman gets an orgasm from shining the kitchen floor," she wrote.

But judgment lingers. A 2013 University of Illinois at Chicago study of roughly 24,000 college students found young men are more likely than their female peers to hold the traditional double standard. The majority, however, didn't subscribe to the old beliefs: 66 percent of men and 70 percent of women said they didn't sex-shame based on gender. (Some indicated they thought promiscuity was off-putting regardless of your sex.)

Carrie Preston, director of the Women's, Gender and Sexuality Program at Boston University, thought Obama's statement showed how far the country has come since just 1998. She recalls the Monica Lewinsky scandal, a high-profile example of sex-shaming. Sure, Bill Clinton caught flak when the public learned he had engaged in a sexual act with a 22-year-old intern. But Lewinsky, the much younger person with far less power, became a target of global scorn.

Lewinsky addressed the issue last year in a Ted Talk.

"Now I admit I made mistakes -- especially wearing that beret -- but the attention and judgment that I received -- not the story, but that I personally received -- was unprecedented," Lewinsky said. "I was branded as a tramp, tart, slut, whore, bimbo."

Preston was a college sophomore when the news broke. "I remember her shaming," she said. "It was a formative thing. Here she was, this young woman, being shamed by the president, the country and ultimately the world."

The lesson she internalised: Men get to recover from a high-profile affair, from the world peering into their sex lives. Women are forever branded.

Talking about the double standard raises awareness, the first step to breaking it, she said. Still, it plays out in how we respond to the presidential candidates' intimate pasts.