I have proposed that polygamy is contrary to natural law, but was given a dispensation (an “exception”) for the OT patriarchs due to the necessity of increasing the population of the Israelites. This dispensation was possible because polygamy was not condemned by divine law (the commandments in Sacred Scripture) until after the time of the patriarchs (when Jesus restored marriage as permanent and between one man and one woman).
It is not clear that we are capable of knowing, or even need to know, exactly why God allowed things to unfold in this way such that polygamy would have been necessary at some point. We can only speculate, and the most reasonable speculation seems to be that it has to do with some historical-social implication for how human nature propagates itself through procreation (e.g., the Israelites needed to be increased in population).
It is not clear that, if polygamy was necessary, therefore so was incest. These refer to very distinct things, even though they both share in the common the result of procreation. In my opinion, incest is more unnatural than polygamy, both because there is a greater natural repugnance to it, but also because incest potentially infringes upon the duty to honor father and mother whereas polygamy does not. It follows that incest is a distortion that has a greater need of being rejected sooner than polygamy.
It is not clear that, because adultery was applied unequally to husband and wife in the Old Law, therefore monogamy was not expected. For example, it might be the “hardness of heart” which Jesus referred to that explains the unequal application of adultery to both husband and wife. As a result, the unequal application would have opened the door to polygamy, but would also not have hindered monogamy as being still the expectation.