Kinists oftentimes argue that Esau's marriage is condemned in the Bible because he had married women of a different race. As is always the case, all of the historically important contradict their interpretation.
Which were a grief of mind unto Isaac, and to Rebekah. The marriage of them itself was a trouble to them, it being contrary to their will that any of their children should marry with the Canaanites, and those the worst sort of them, the Hittites; it having been the care of Abraham, the father of Isaac, that his son should not marry with them, and laid a strict injunction on his servant not to take a wife for his son from among them; and which was an example to be followed in later times, and which Esau very likely was not ignorant of: and besides this, the women themselves he took for wives were very disagreeable on all accounts, partly because of their religion, being idolaters, and partly by reason of their temper and behaviour, being proud, haughty, and disobedient; as all the three Targums intimate.
Adam Clarke explicitly denies that the reason for the grief caused by the marriage had anything to do with the marriage itself and instead says that it was
by their perverse and evil ways, brought bitterness into the hearts of Isaac and Rebekah. The Targum of Jonathan ben Uzziel, and that of Jerusalem, say they were addicted to idol worship, and rebelled against and would not hearken to the instructions either of Isaac or Rebekah. From Canaanites a different conduct could not be reasonably expected. Esau was far from being spiritual, and his wives were wholly carnal.
Both commentators mention the Targums, which explicitly attribute the cause of the grief to the irreligious character of his wives. So also Matthew Henry:
Esau was foolish in marrying two wives together, and still more in marrying Canaanites, strangers to the blessing of Abraham, and subject to the curse of Noah. It grieved his parents that he married without their advice and consent. It grieved them that he married among those who had no religion. Children have little reason to expect God's blessing who do that which is a grief of mind to good parents.
Calvin's commentary, as usual, is instructive:
For many reasons Moses relates the marriages of Esau. Inasmuch as he mingled himself with the inhabitants of the land, from whom the holy race of Abraham was separated, and contracted affinities by which he became entangled; this was a kind of prelude of his rejection. It happened also, by the wonderful counsel of God, that these daughters-in-law were grievous and troublesome to the holy patriarch (Isaac) and his wife, in order that they might not by degrees become favorable to that reprobate people. If the manners of the people had been pleasing, and they had had good and obedient daughters, perhaps also, with their consent, Isaac might have taken a wife from among them. But it was not lawful for those to be bound together in marriage, whom God designed to be perpetual enemies. For how would the inheritance of the land be secured to the posterity of Abraham, but by the destruction of those among whom he sojourned for a time? Therefore God cuts off all inducements to these inauspicious marriages, that the disunion which he had established might remain. It appears hence, with what perpetual affection Esau was loved by Isaac; for although the holy man justly regarded his son's wives with aversion, and his mind was exasperated against them, he never failed to act with the greatest kindness towards his son, as we shall afterwards see. We have elsewhere spoken concerning polygamy. This corruption had so far prevailed in every direction among many people, that the custom, though vicious, had acquired the force of law. It is not, therefore, surprising that a man addicted to the flesh indulged his appetite by taking two wives.
Calvin explicitly makes the point that the reason the marriages Esau had contracted were unlawful had to do with the fact that God had promised the land to Abraham, which would entail the destruction of the descendantst of the Canaanites (among others; see Gen. 15:12-21). He also makes it clear, with the other commentators, that the Canaanites were "reprobates" in God's eyes, and for this reason lacked the religion of the Jews. Calvin, in the last sentence of the quoted commentary, likewise alludes to the same fact that the author of Hebrews does, as with some of the other commentators quoted; namely, that Esau was sexually immoral for engaging in polygamy, which is contrary to God's law.
The reasons we thus see for the unlawfulness of Esau's marriage are as follows:
1) Polygamy was sinful.
2) Many commentators suggest that the character of the wives whom Esau had married was bad, and that they engaged in idol worship.
3) God had promised the land of Canaan to Abraham's offspring, which would entail uprooting the inhabitants of the land. Esau's wives were inhabitants of this land, and their descendants were to be uprooted thus. It was therefore imprudent of Esau, for this reason, to marry such women.