Anomalies exist in biology. For example, in most mammals the male is bigger, but in hyenas the female is bigger. Dwarves' monogamy could also be like this.
The problem is that females leading harems of males in any mammalian species is unlikely given the nature of reproduction, hence why it has never existed through a whole society in the way free love or male led harems have. Females cannot produce vast numbers of descendants in the way males can, so dominant females would not have far more offspring from having more lovers and so their genes would not be passed on more. There are sexually liberated societies where both females and males can have many lovers, like the Mósuō people, but there are none where harems of men are common because of the nature of mammalian reproduction; a female cannot produce more offspring with many lovers than she can with one, unlike a male with many lovers.
Dwarven sexuality should either be free love or monogamy, since they do not have gender double standards. Civilised humans should have patriarchal systems where many males have mistresses while women are expected to be monogamous. Elven queens can have harems of male officials (look at Catherine the Great for a real example of this), but this cannot exist all the way through their society for the reasons I have already given (their reproductive rates would be very low if only some women are being impregnated, and having multiple lovers does not boost females' fertility).
Essentially, dwarves should be gender neutral, humans patriarchal and elves matriarchal. However, because of the nature of mammalian reproduction, elves cannot have harems of men all through their society because that would severely damage their reproductive rates, so below the queens and governors their society should either be a matriarchal monogamy (which is possible, monogamy is not innately sexist) or free love model. Of these, only 2 are currently implemented.