This is actually untrue. It seems unrealistic because we live in a modern world with a Christian/Jewish monogamous standard, but throughout history polygamous cultures were the norm, and monogamy the exception. The Arabs had multiple wives. The Indians had multiple wives. The Chinese even recognized the emperor's harem as a political institution. So did the mongols. Southeast asia. Vikings....
So yeah. It only feels unrealistic because we live in a world where European Christian cultures pulled ahead in the technological and development race and spread more than any other.
Besides, "powerful man gets many women" is a stereotype for a reason. It happens. I'd argue you should see it as more of a life lesson than a mere writing cliche. I'd agree that is far from how many trashy harem novels picture it to be - less happy lovey dovey, more backstabbing and competing for attention - but the idea in and on itself not unrealistic or illogical