"You seem to be conflating access to other resources with access to partners."

While these are distinct matters, they are closely connected.

"Artificially enforced monogamy and celibacy doesn't redistribute resources of any kind in the modern world."

Alimony certainly does redistribute resources.

"There are still beta males and females so long as people can choose."

True - there are males and females who cannot get the partner they prefer, and therefore face the choice between celibacy or working matters out with second preference partners. But enforced monogamy means beta females cannot flock to the harem of alpha male, and for that reason need to look at beta males.

"And humans aren't lions. Large harems have been an odd idea that only occurred to a few cultures, and which seems to impede progress. "

No. Large harems are common and a sign of progress. Among egalitarian hunter-gatherers or headless farmers, most adults are in breeding pair marriages and rarely can one husband afford to support two or three wives by his sweat or smarts. But in societies which have large social units and where a few men can rule and command the resources of many - whether you are looking at Near East or Far East, Africa, Mesoamerica or Andean region, you will find men who will apply their resources to get sexual access to large numbers of women and beget many offspring.

It is normative monogamy, where men who could afford to take multiple wives are not allowed to, that is an odd idea of the Greeks.