It's Orwell so it's well written.
But even Orwell needs references to back up this argument.
See: http://www.dailymail.co.uk/femail/article-481882/Condemned-v...
Also, while the man is out at war, the woman usually won't be procreating with anyone else until the man dies. The news of his death will probably affect her badly, and procreation is probably the last thing on her mind at least for a while (after which she may be too old).
But yes, hard data would be even better.
Beyond cultural acceptability, there is objective optimality. Since men thrive to validate themselves by providing more than they consume (cf. article again), it's a comparative disadvantage for women to raise kids without the benefit of that surplus.
In a society where it's reasonably easy to secure a (male) provider, you're a major loser as a woman if you can't secure one. In a society where it's hard, e.g. due to skewed gender demographics, it's acceptable to be a single mom.
There's been a quite politically incorrect paper about black US communities, their lack of men due to huge imprisonment rates and higher mortality, and the resulting prevalence of the single mom model. I think it was referred to by Freakonomics.
There is some counter-evidence to this claim: http://en.wikipedia.org/wiki/Dear_john_letter
"Dear John," the letter began. "I have found someone else whom I think the world of. I think the only way out is for us to get a divorce," it said. They usually began like that, those letters that told of infidelity on the part of the wives of servicemen... The men called them "Dear Johns".
Or don't get caught....
Anyway, reproductive success (or just the chances of getting a wive) are even more skewed in polygamous societies, than in monogamous ones. It's an alpha-males paradies, and a loser's hell.
"Why is it worse to kill a woman than a man? The argument usually advanced is that in killing women you are killing the breeders, whereas men can be more easily spared. But this is a fallacy based on the notion that human beings can be bred like animals. The idea behind it is that since one man is capable of fertilizing a very large number of women, just as a prize ram fertilizes thousands of ewes, the loss of male lives is comparatively unimportant. Human beings, however, are not cattle. When the slaughter caused by a war leaves a surplus of women, the enormous majority of those women bear no children. Male lives are very nearly as important, biologically, as female ones."