That's a fairly self-serving post. When men took on many wives, no matter what the culture, it was mostly wealthy/powerful men who did so. The reason was that the economy was such that if a woman wanted to survive and her children to survive, she needed to partner with a man who had wealth. After all, daughters were given away by families for they did not bring wealth to them, so these women were looked upon as liabilities by their families whereas the sons often brought home the woman's dowry and her resource as another household servant.
This is nothing to say of her love life, for though she was expected to provide sex to him and be loyal only to him, what she felt inside with regard to romantic love is not known. Yet she was probably forced to arrange her priorities such that mere economic survival mattered most. This pattern was the same in recent Arabic, recent Asian, current African and ancient European culture. This was not "evolution", nor was it "natural". It came out of sheer necessity as forced upon by an agrarian/serfdom economic system. That system gave advantage to men because it gave advantage to physical prowess and the ability to be mobile (women were much less mobile because they bore children for nearly a year at a time). Today the economy is completely different. Because of it, women can now do most things that men can, so they seek equality. It is the attainment of economic parity that allows them to demand a more fair transaction (ie: equal loyalty) from their partner.
It is true what you say of older women finding their spouses cheating on them, divorcing, getting a settlement, and being miserable. I've known a few myself. Not much happiness can be had in today's economy when assets are split, even though a woman may fool herself into thinking that economic independance through divorce is the path to it (happiness). The problem is the woman's expectation of her own economic equality, idealistic notions of romance, and expectation of attractiveness to future mates at a more advanced age. The truth is that though society (primarily the feminists) have emphatically sold them on the idea that they can now expect economic equality, it is otherwise. Even in today's economy, an older woman's prospects of becoming wealthy and retaining it are much worse than a man's, and because of this, economic necessity once again becomes the motive for finding a partner as it did in ancient times. She seeks a man who can provide for her, and that's what she will find attractive as those in generations past. And she will also find the same conundrum as those previous to her, that men are primarily attracted to one more youthful to them. Her prospects of finding the wealthy man are thus diminished, and she will have to reconcile the greatest probability that she will end her days poor and alone.
Today's economy demands two fully working and capable people for survival and for savings for a suitable retirement. It is no longer realistic to expect that the average man can be the sole provider. As more couples accept the notion that a partnership is first and foremost a friendship rather than a romance, they will consider that should the romance end, the friendship can continue. I'll bet that in the future you will see more divorced couples co-habitate rather than split assets, for example, splitting the family home to upstairs/downstairs with each one living in their separate suite. They will then be free to pursue romantic inclinations independantly of each other and still retain their collective wealth.