These were Millennial undergrads, and mostly from middle or higher class backgrounds, so they couldn't see that in a different environment than the one they grew up in, staying with a boyfriend or husband who slaps you around once in awhile may be the lesser of two evils -- namely, compared to the far greater exposure to violence that she'd face if she didn't have a protector, someone who had at least some incentive to watch out for her (like having someone to sleep with).
To put more real-world detail onto that generalization, let's have a look at a young girl who kept choosing real bad boys as her partners. It's from a 1990 ethnography, Runaways: In Their Own Words: Kids Talking about Living on the Streets. The subjects were people living in a Los Angeles shelter for teenage runaways, and the interviews were conducted in 1988. Ally, a 16 year-old white girl from Louisiana, describes her home life when she was a child:
My mom got back into heroin, and she started drinking heavily too, and that combination got her up for beating on me. It took her a couple years to get like that but my stepdad had been beating on me ever since I can remember. . . I can still remember how [her stepdad] Frederick used to kick me down the hallway with his steel-toed boots -- his biker boots. Some dads send their kids to their rooms, but my stepdad kicked me to my room.
After she pilfered one of his candy bars, he went to beat her, and when she tried to break away, he threw her on the ground and messed up her ankle:
That was the only time I went to the hospital, and that was the last time I sneaked one of his candy bars out of the freezer.
When she was 14 and in a foster home:
A few weeks after I got there, I met some bikers who were hangin' around the neighborhood, and I ran away so I could hang out with them. And then I did some serious drugs. These guys were all about three hundred pounds and real hairy; and they did lots of drugs.
Why did she stay around guys who robbed for a living and clashed with other biker gangs?
I stayed on with 'em for a month though, because they took care of me. Whenever anyone messed with me I just ran to [her biker boyfriend] Peter and other boys and they took care of it. Once I was hanging out in this big parking lot where all the young people who did drugs would go to, at night, to do drugs or sell drugs and talk about drugs. [Laughs.] This guy came over to me and talked some shit to me, and then he started trying to kiss me. I pushed him away, you know, and then he was yelling, "I've seen you around and I know you're a sleazy bitch, so what's stopping you now?" He just kept on with the shit.
Lucky for her, someone had her back:
I went to a phone and called to a bar where I knew Peter was at, and I told him what the guy said and everything. About five minutes later Peter and one of his friends come cruising in on their Harleys -- they just jumped off their bikes, asked me which one of the guys it was, and beat the shit out of him. Those guys were the best protection you can ever have. No one would ever want to mess with them.
Although she didn't explicitly say so, her boyfriend sounds like the kind of guy who probably exploded at her verbally now and then, and probably hit her when he lost his cool. But even if it happened, she did not find it worth mentioning in comparison to the larger threats she faced from her stepdad, her drugged-out mom, and random strangers trying to push themselves on her in an evidently lawless area.
A boyfriend or husband has a sexual incentive to dial down the violence and protect her from the violence that others would do to her -- he wants a partner who he can continue sleeping with. Other abusers have a more financial incentive, like pimps who occasionally rough up their prostitutes, but who want to keep them in a certain degree of health because otherwise they can't work and bring him any money. Again, try putting yourself in her shoes and see if maybe the pimp's abuse isn't the lesser of two evils compared to the sick shit that a john might carry out, whether physically or psychologically.
In a 1981 fictional account of teenage prostitutes that was however based on the author's fieldwork, Children of the Evening, case worker Trudee recognizes a murder victim in the newspaper as one of her former girls. She then asks Frank, the policeman assigned to the case:
"Who did it? Who cut off that girl's legs and slit her throat?"
"We don't know, Trudee," he answered softly. "There's plenty of sick ones out there. They haven't found her legs."
"I know, Frank. Oh God, I don't feel so hot."
For a few seconds the line was silent.
"Frank, this had to be a sick, crazy trick. It doesn't sound like a pimp's style, does it?"
"No, I think it was a trick, definitely."
We both knew that pimps use mental cruelty and beatings to teach their girls a lesson, but when they kill, it's more often by accident.
"Sex and murder always make me think of tricks, Frank, not pimps."
All those potential and actual tricks could care less if a prostitute gets hurt or dies after he's had his way with her, so any of them that's prone to violence won't hold back at all -- and you can imagine what type of guys she's likely to see. That's a far greater danger than the pimp, who wants to see that she's healthy enough to keep working.
So, a closer look at the different world that these females live in makes their choice to stay around abusive males look not so stupid or self-destructive, given the more violent alternatives they face without some kind of protector.