a) Adult male civilians are somehow legitimate targets - or at least more legitimate. Be they 95-year-old wheelchair-bound asthmatics or hale conscientious objectors or simply greengrocers trying to sell sprouts, their Y chromosome means that they are all combatants in potentia. So it's okay(ish) to kill them, on the same principle that it's okay to castrate men because they're all potential rapists - right?
b) Women are incapable of offering any kind of threat. Despite the many thousands of female members of the professional armed forces, and the large number of female suicide bombers for that matter, we know that they can't be real soldiers - it's just not in their feminine natures. Not least because...
c) ... the phrase "women and children", while infantilizing women through its implied apposition, also reminds us that the archetypal - the essential - woman is a nursing mother. This is the condition to which all other women aspire, or of which they fall sadly short. Whereas "men and children" - well, that just sounds a bit odd, doesn't it?
In fact the whole thing seems the product of a rather toxic mess of unexamined sexism. I was reminded of this the other day when half listening to Andrew Marr's Start the Week, which featured the Archbishop of Canterbury and Philip Pullman, amongst others, debating religion and morality. Someone (I can't honestly remember who) suggested that, despite their differences, they could all agree on certain universal moral principles such as that it was wrong to kill women and children. No one demurred, or suggested that this might actually be a pretty culturally-specific principle. Of course, I realise that I would have looked foolish shouting "Kill the women and children too!" or "But what about the menzzzz??" at the radio; and it's hard to object without looking as if one is doing at least one of these things. Also, I agree that it is wrong to kill women and children, so what's my beef? All the same, I suspect that had someone averred as a universal moral principle the equally-true idea that it's wrong to kill white people he would have got some pretty funny looks.