https://pubmed.ncbi.nlm.nih.gov/10902048/
This paper seems to disagree heavily with you.
And extrapolating this to the middle ages and earlier:
In general, countries with a lower overall life expectancy had a lower number of women per 100 men aged 75+, while countries with higher overall life expectancy had a higher female to male ratio in this age group. A hundred years ago there were nearly equal numbers of women and men aged 75+ in many countries.
It doesn't seem to disagree with me at all.
It feels like you're right, but what "feels right" turns out to be wrong pretty often.
Off topic, but such archaic means of warfare still confound me. Imagine being some 16 year old kid standing in the front line facing charging armored knights in the Middle Ages. Or staring across from another line of musket/riflemen and being expected to eat a volley of musket balls while standing straight and unmoving.
That doesn't necessarily imply polygamy, and it doesn't necessarily imply "incels". Imagine a society in which women get married at age 16 and typically die during childbirth before they reach the age of 32, and men get married at age 32, if they live that long, which most of them don't, and then get married again when the first wife dies during childbirth. I'm not saying that's how it was, but some societies were a bit like that and I think you could probably get the 80% and 40% numbers with a set-up something like that.