Two years ago, the University of Pennsylvania campus broke into a storm of protest after an off-campus fraternity circulated an email invitation to one of its parties, featuring this lovely piece of poetry:

May we have your attention please
We're looking for the fun ones
And say f— off to a tease
Wednesday nights will get you going
With bankers flowing all night
Tonight is your first showing
So please wear something tight

Outraged students posted hundreds of fliers around campus, reproducing the email under the headline "THIS IS WHAT RAPE CULTURE LOOKS LIKE."

University officials also denounced the email, and then did what university officials do: They convened a task force. The offending fraternity wasn't part of Penn's official roster of Greek organizations, which made it especially difficult to regulate. So the task force recommended new rules for all student activities, whether they occur on campus or not. Today, students are required to register all social events beyond a certain size with Penn and hire university-sanctioned bartenders and security guards.

But a few months later, the campus newspaper reported that women were still going to parties at the fraternity that had provoked the controversy. Attending such events was a "personal choice" for women, who "should be allowed to put themselves in the social spaces they are comfortable in," as one female student told a reporter.

But those personal choices have social consequences, fueling an institution that has done more to perpetuate sexual violence — and sexism, period — than anything else on campuses. And the only way it will stop is if women stop going.

>> READ MORE: My son died from hazing, but banning fraternities won't solve problems with Greek life | Opinion

The time has come for women to boycott fraternities.

It isn't just that fraternity men are three times more likely than other men on campus to commit sexual assault, as one study claims, or that women who attend fraternity parties are 1½ times more likely to be victims of assault.

And it isn't just that 30 percent of campus incidents in which students are drugged — a common prelude to sexual assault– occur at fraternities. That's what allegedly happened in February at Temple University, which closed a fraternity last month while it investigated reports by students who believe they were drugged there before being assaulted.

Instead, to quote the Penn protesters, it's about the culture that fraternities embody.

From their birth in the 19th century, fraternities have sought to distinguish "real men" from the rest. And in the 20th century, when coeducation kicked in, one of the things that distinguished a real man was a demeaning attitude toward women.

Some fraternities barred new pledges from talking to females, even in class. Others encouraged members to have sex with as many women as possible. To this date, some fraternities require brothers to report their sexual conquests at weekly meetings.

Meanwhile, hazing rituals — like the one that killed Pennsylvania State University student Tim Piazza last year — required pledges to drink copious amounts of alcohol, then submit to a range of humiliations. Anyone who demurred was deemed a "pussy," which is the worst thing that a real man can be.

"The popular American macho image, the incredible terror of being thought a pansy, or worse, a faggot, impels many men to support hazing activities as consistent with masculinity," explained a member  in 1978 of Sigma Alpha Epsilon (SAE), the nation's largest fraternity.

Around the country, college women have their own nickname for SAE: Sexual Assault Expected. According to journalist John Hechinger, 15 of 230 SAE chapters were reported for sexual assault between 2011 and 2016. Two chapters were the subject of five separate reports of sexual assault or misconduct in 2014 and 2015 alone. And over a single Halloween weekend in 2014, rapes were reported at SAE parties in Maryland, Georgia, and California.

At Yale, meanwhile, campus protests were triggered in 2015 when students of color were allegedly turned away at the door of an SAE party with the rebuke, "We're only looking for white girls."

But the chapter had already been kicked off campus for a pledge session in which several members boasted about having sex with the same female student, who described their humiliation of her in the school newspaper.

So why were women lining up to get into the off-campus SAE party — or to any fraternity event, for that matter? When I put that question to my own female students, they usually reply: That's where the parties are.

>> READ MORE: As an Alpha Kappa Alpha, my Greek life has been about help, not harm | Perspective

That's always reminded me of Willie Sutton's famous quip, when asked why he robs banks: That's where the money is. And it doesn't absolve students of responsibility for their own choices any more than it excused Sutton.

Let's be clear: The people who are at fault for sexual violence and humiliation at fraternities are men, not women. But anyone who goes to a fraternity party is complicit — to borrow a favorite term of the #MeToo movement — in what happens there. It won't do to rely on university officials, who have stepped up their regulation of fraternities at Penn, Penn State, and many other schools. Students have to step up, too, by voting with their feet. The only thing that will change the sexist culture of fraternities is for women to stay away from them.

Jonathan Zimmerman teaches education and history at the University of Pennsylvania. He is the author of "Campus Politics: What Everyone Needs to Know" (Oxford University Press).