This sounds like Raiders from the Rings (1962) by Alan E. Nourse.
A young adult novel in which the human race is divided into the Spacers and those who still remain on earth. There is a constant low-level war between the two groups, with the Spacers raiding earth for supplies and for women. The war turns hot in the book and three teenagers, one Spacer and two from Earth, must find a way to bring peace.
The story opens with a Spacer raid on an Earth transport ship where all the crew are distracted by a mauki singing a song of loss:
The raiders could hear the mauki's chant from the moment they boarded the ship from Earth.
It came from somewhere deep in the heart of the craft, and they paused as soon as the outer hatchway had been forced, listening in spite of themselves in the darkness of the corridor. It came to them softly at first: a clear, sweet woman's voice, sharp as crystal in their ears. Then it rose higher, mournful and shattering, and the words became distinct in the ancient, heart-rending lament that the raiders had heard so many times before. Urgent as their mission was, they could not help but listen for a moment, feeling the wave of sadness and longing surging up in their throats.
Beyond them the ship's corridor was empty; there was no sound here other than the chant. Not even the throbbing of the ship's generators was audible in the blackness. The raiders stood transfixed for a moment. Then Petro, the leader, took a deep breath and flicked on the battle lantern at his belt.
"She's here, all right," he said. "She must have the whole crew listening. Let's go."
[...]
The mauki had been huddled in the corner of the cabin, sobbing. Now she looked up, tears still streaking her face. "What are you going to do?"
"What do you think I'm going to do?" Petro said harshly. "They're butchers. Kidnaping you is one thing. Murdering a five-year-old child is something else. Well, they haven't even go their battle lights on yet. We'll gut them."
The woman was on her feet. "No, please! Let them go back home."
"So they can murder more of our children?"
"You don't understand. They were afraid of him."
[...]
The Earth ship knew, of course, what the raiders could have done. Every man in the crew knew that, from the captain down, and no one could understand why they had been allowed to escape. Yet in their minds the haunting chant of the captive woman still echoed; they could still hear
her song of longing and loneliness. Back on Earth they would remember those words, and talk about that song for years to come.
And that was what the mauki wanted.
Kidnapping women is part of the program:
"Even that's been taken care of," Ben said eagerly. "The word has been leaked that our strike point will be a South American food dump. And they've garrisoned that one to the teeth and pulled most of the guard strength away from the real objective."
"And what about women?" the older man said.
"Well, there's that on any raid, naturally," Ben said. "But the last raid filled the quota pretty well, so only the firsttimers are expected to bring back girls this time." He shrugged disgustedly. "Matter of fact, that's all my personal orders will let me do on this raid... find a good mauki prospect and haul her back here. But you already know all this. The Council has the whole plan from the Raid Commander. Why are you asking me?"
"To see if you know what you're walking into," Ivan Trefon said.
"Well, there's nothing very exciting about kidnaping a girl," Ben admitted. "But on the next raid they'll let me do more."
The Spacers began as soldiers who refused to destroy the Earth by bombing it and were exiled for their conscience:
They realized that they held in their hands weapons that could wipe their home planet barren of life. At first as individuals and then in frank conspiracy they realized that these weapons must never be used. So, when the moment of truth arrived in the councils on Earth, and the Earth forces delivered their blows at each other, expecting the massive backing of their garrisons in space, the men in those garrisons drew together shoulder to shoulder and withheld the devastating attack they were expected to deliver.
There had surely been a conspiracy, Ben Trefon thought, but a conspiracy to draw the teeth of the warring factions on Earth. The Earth councils had raged and threatened and pleaded, and finally had gone on to fight their war as best they could, but its force was blunted as the space garrisons refused to deliver the suicidal blow. After the dust of the war had settled, those brave men in space reaped the reward of their deed as the councils on Earth turned against them in frustration and hatred. It was a bitter reward, and time did not change it. Branded as traitors, they were exiled from the planet of their birth, driven back when they attempted to come home, forced to take up a lonely, wandering life in the great emptiness of space beyond the boundaries of Earth.
They have no daughters:
"I can't imagine how I'd feel, no matter how hard I try. You see, I've
never had a sister, and neither has any other man born in space. Not one
of us. Never."
[...]
"In space the single X chromosome that men carry is damaged. Our scientists still don't know how, exactly; some of the genes in the X chromosome are just put out of commission, so that the X behaves like a Y. And as long as Spacer men can provide only Y chromosomes, they can never father girls, only boys."
The aliens are trying to prevent the extinction of human life:
The creature hesitated. "We cannot read the future. We can only predict on the basis of long and bitter experience. Should your war be pursued to its end, the odds are four to one that all human life in the solar system will be obliterated, that the spark will be extinguished once and for all. And that cannot be permitted to happen."
[...]
"Because that is our purpose here: to care. We have watched your planet for millennia, since the first spark of intelligence flared up in your people, and we have watched that spark grow into the raging fire it is today. Our job is to keep that fire alive until you cease being children and learn how to control it yourselves, until you learn how to use it. Then our work here will be done, if you have not destroyed yourselves in your childishness before you can mature.
