sarcasm, polygamy and monogamy have the same major social problem, one spouse too many.

A significantly-greater-than-1.0 male-to-female ratio would be a different cause of the same problem; polygyny produces it even without that ratio difference. (Polyandry would seem, in principle, to offset it, but there aren't lots of historical examples of that to study.)

Wonder what the polygamy male-female ratio is compared to current societies with skewed ratios (CN / IN)?

No. What seems to matter in the study is whether men have acceptable marriage prospects. (It'd be interesting to see, in that context, whether permitting same-sex marriage reinforces the effect observed and further reduces the same problems. It'd also be interesting to see, as marriage itself becomes less of an expectation, whether the effect is more from men being married -- in which case denormalizing marriage would be expected to reverse the benefits -- or whether the effect is more from men not being alienated by being unable to meet a social expectation of marriage -- in which case denormalizing marriage would be expected to reinforce the benefits.)