When Locke mentions polygamy in his writings, he does not condemn the practice and even seems to endorse it under certain conditions. This attitude is out of step with that of many of his contemporaries. Identifying the philosophical reasons that lead Locke to have this attitude about polygamy motivates our project. Because Locke never wrote a treatise on ethics, we look to a number of different texts, but focus on An Essay Concerning Human Understanding and Essays on the Law of Nature in order to outline his basic ethical theory. We argue that this theory, the elements of which include moral mixed modes, the law of nature, and the comparison of these modes with this law, is broad enough to accommodate practices such as polygamy. Our interpretation shows that Locke's line of thought on marriage is strikingly flexible for the seventeenth century and even compared to some public debates on marriage in our time.