Anyone who thinks this stuff is straightforward has an incredibly lucky life, and a very lacking imagination.
SO MANY of the women you're taking about will have said before the relationship that 'if a man did [X] I'd just leave'. And then when you're 35 weeks pregnant he does - but you're due so soon, and he's sobbing with guilt and saying he wants to die, and you know how he can be really kind and how he's had to grow up with abuse himself, and for the next weeks he's really trying, so kind to you and not even drinking - way better than your own dad, who never even felt bad about it. Your mum says so too, and she's worried about how you'll cope as a single new mum, so you agree it's out of character and stay.
Then it happens the second time months later, and you think about going, but he's been really stressed because the baby's not sleeping, and you're on statutory maternity pay and have run through your savings, and you're also really scared that if things got mean, your post-natal depression would be brought up in court and you'd lose your baby. So you stay.
Then for a while he's got a new job, and for a good while it's just like the old days, and suddenly you're pregnant with #2. He's drinking again in the evenings and it makes him short tempered, but bad morning sickness means you do appreciate him 'helping' with your toddler, who also absolutely idolises him. Your work doesn't take your frequent absences well though, and they tell you that your role is redundant (they say it's nothing at all to do with pregnancy, of course). He loves having you home, and has loads of expectations and advice about how you need to care for it and your kids, and isn't keen on visitors, and gets really anxious and grumpy when you go out, so your world is getting smaller. Still no savings as he sees all your spending. Hard to go back to work, as he points out you'd barely be making £30 a week after childcare, and it's not like he can help much with the school run.
You think about leaving sometimes, but you couldn't afford a house nearby, and he'd pay for a good lawyer while you don't have any savings, your eldest adores him and would be devastated - also his temper is bad enough that you think maybe it's better if you're there to keep things calm. You're not perfect either, sometimes you've called him names too, and he says you're the abusive one, with your control-freakery about housework and drinking. Maybe if you're a better wife, this will sort out.
He's charming, none of his family or mates would believe it, and he has the money to get legal help. You'd miss him, too, in between the bad moments he makes you laugh and you didn't go into marriage to be like this. He didn't either, you know he was damaged himself. And that's before all the logistical and social problems that terrify you about being a single mum.