The Soviet Union, having suffered some of the worst casualties during the war, developed a de facto system of bigamy. Although not encouraged in law or by the authorities, men having both a legal married wife and an "army wife", ie a girlfriend became a very common practice towards the end of ww2 and the years after the war and generally accepted as a fact.

Daycare for children was instituted for many widows that had to work since their men had died fighting for the Soviet Union during the war.

There were pre-war a tradition in many countries to retain a daughter (not allowing her to marry) to care for the elderly in the family - this tradition soaked up at least a small part of the surplus young women.

Edit: Sources.

http://web.williams.edu/Economics/faculty/brainerd-rfwomen.pdf

I cannot speak to World War II here, but there was recently an article on the legacy of World War I on British women.

By the time the Armistice was signed in 1918, a British woman aged 16-32 stood only a one-in-ten chance of marriage. The 1921 census revealed that there were two million “surplus” women of marriageable age, a statistic that led to publication of a pamphlet, “The Problem of the Surplus Women.”

The author argued that not only was the issue of surplus women extremely valuable for women asserting their rights in the workplace, they were also forced to form their own community outside of the expectations of marriage and children.

The Great War was notorious in its ridiculously high casualty rate, so I think it an even better example than World War II of what happens to women when they are unable to marry at all.