If prostitution of children is illegal, why is it that we allow an estimated 100,000 underage girls and boys to be sold for sex in America each year — many on a single U.S. website, Backpage.com?

That’s a reflection of law enforcement priorities, but several brave girls who allege that they were pimped on Backpage are trying to change them. They are fighting back in lawsuits that could have far-reaching implications for sex trafficking in America.

Two young women who say they were each sold on Backpage at age 15, and raped hundreds of times as a result, are suing the company in Boston in federal court. Another suit is winding its way through Washington state courts, pursued by three girls who say they, too, were sold for sex on Backpage — in the case of two of them, when they were 13 years old.

The girls in the federal suit are represented pro bono by a major Boston law firm, Ropes & Gray, which has five lawyers on the case. The suit charges that Backpage has “perfected a business model that profits substantially from aiding and participating with pimps and traffickers in the sexual exploitation of children.”

“When on Backpage, I was advertised in the same way as a car or a phone, but with even less value than a bike,” says one of the girls who is a plaintiff in the federal suit. “Men would view their options, and if I seemed like the best one, they would call.”

Now 17, she says she was sold for sex on Backpage when she was 15 and 16; she estimates that she was raped 1,000 times as a result.

I first wrote about Backpage a few years ago when it was used to advertise a 13-year-old girl being enslaved in Brooklyn, N.Y. One day the pimp dropped her off at an apartment building and waited at the entrance to make sure she did not run. She hurt too much to endure another rape, so instead of going to the apartment that had ordered her, she randomly pounded on another door and begged to use the telephone.

She called her mother, and then dialed 911. The pimp is now in prison, but Backpage profited on the ad — as it always does.

Attorneys general from 48 states have written a joint letter to Backpage, pleading with it to stop exploiting children.

Liz McDougall, the lawyer for Backpage, declined to comment on the allegations in the lawsuits, but she told me: “We remain committed to effective measures of prevention and successful prosecution of this heinous crime.”

That’s absurd. Backpage claims to report possible sex-trafficking cases, but Yiota Souras, the general counsel of the National Center for Missing and Exploited Children, says if parents report to Backpage that their teenage daughters are being sold on the site, the company doesn’t always remove the ads or prevent new ads for the children from being posted.

The lawsuit says Backpage floods the authorities with reports of possible underage girls to pretend to be helpful, while actually impeding the effort. Meanwhile, Backpage refuses to use screening software that might actually detect ads for underage girls.

Americans rightly waxed indignant at the way the Roman Catholic Church or Penn State turned a blind eye to the sexual abuse of minors. But our entire society does the same thing.

Isn’t it time to stop?