Let's mainly focus on the UK and France, but if someone has information about other belligerents.
Millions of male casualties, female casualties are probably a fraction thereof. Millions of wives widowed, millions of unmarried girls suddenly finding it hard to find a husbands (sexual mores were more restrictive than today, friends-with-benefits was not really on the table).
My question is primarily, did millions of women resign themselves to living with cats, or did society become informally more polyamorous, polygamous, i.e. a lot of married guys had mistresses on the side?
So...I'm no expert, but I'm fairly certain you're overstating the number of dead during the First World War. There were millions of dead in total but no one country had multiple millions of dead men / widowed women.
Look at the two "biggest losers," France and Germany. If memory serves** about 1 million Frenchmen were killed in the war. But that's out of a total population of nearly 40 million. Nearly 2 million Germans died, but that's still out of a population of more than 60 million. I don't want to sound like I'm minimizing the casualties...but I don't think that enough men died that there was an outbreak of spinsterhood in the years after the war.
That said, The Flapper culture of the 20s was considerably more sexually liberated than the pre-war Victorian ideals. I've always read that the excesses of the Roaring 20s was mainly a reaction to the bleak austerity and disillusionment of the War-years, but I suppose a slight dearth of marriage-aged men could have been a contributing factor?
**I've read numbers right around these in a few books, the latest being Attrition by William Philpott, but I'm going off memory at work. Can somebody can back me up / correct me?
edit: this question was really intriguing, so I did a bit more searching around. I found a pretty interesting little article on the idea of "Surplus women" in the UK after the war published by . You might start here, and check out the reading list for more info?