“So what *is* the reason Hashem allowed men to marry more than one wife (but not vice versa)”
One answer is that shevet and chelek go by the father and it would cause too much confusion if we didn’t know who the father was.
But a man has an obligation to make his wife feel loved and he had to have her permission before marrying a second. And the second wife had to agree to enter a marriage where there already was another wife. It calls her a tzara because it’s telling the man that as much as he is allowed to have two wives it’s going to be huge pain for him.