If a strange pandemic wiped out 70% of men would polygamy become acceptable? Or, if 70% of women were lost, would polyandry be OK? What etiquette would apply in a menage-a-many?

If polygamy or polyandry became necessary and acceptable, I'd buy stock in the company that makes these:

There wouldn't be time for it to become acceptable. When wars killed off a large number of men, the ratio of male children being born increased, to even things out.  So only one generation would be short of males. Just like more male baby boomers were born than female.