Various royal families have, throughout history, made it a practice to marry relatives in order to consolidate power. Are there any stories of relatives refusing to marry because one or both of them were unable to be sexually attracted enough to consummate the union? Was intermarrying practiced primarily only be royal families, or did other families also engage in intermarrying? For example, a high ranking noble family that similarly sought to either retain or increase power/wealth/status. Or even a family of commoners who had attained a certain level of financial prosperity--skilled craftsman, tradesmen, and merchants, etc.
Your question does not actually set a period, which is something the sub does generally require (explicitly prohibiting 'throughout history' questions).
However, I think the specificity of your interests does go so way to ameliorating that part of the question. I would still recommend restricting your scope, even if that is just to Europe in the later Middle Ages / Early Modern period.
I would also recommend defining what you mean by 'intermarriage'. Among the high nobility and monarchical families, Christian marriage was prohibited within seven degrees of consanguinity until the Fourth Lateran Council of 1215. After this point the prohibition was reduced to four degrees.
Marriages to kinsmen within those degrees required the pope to sanction the marriage and demonstrate that there was no-one of near or equal status who could provide an alternative to the proposed or current spouse (as many nobles and rulers, from the twelfth-century, sought papal blessing ex post facto - or used not having it as an excuse to have the marriage annulled).
So think carefully about whether this is the type of thing you are interested in, or are you more interested in Egyptian, Greek, Roman, Mesoamerican, etc. practices.
I have provided a more general bibliography for the English Crown and peerage in the Middle Ages (post-Conquest):
David d'Avray, Medieval Marriage: Symbolism and Society, (Oxford, 2005) | Dissolving Royal Marriages: A Documentary History, 860-1600, (Cambridge, 2014) [there is a forthcoming companion study to complement this sourcebook, although I do not know when it will be published].
J.S. Boswell, Edward III and the English Peerage: Royal Patronage, Social Mobility and Political Control in Fourteenth-Century England, Woodbridge, 2004).
James A. Brundage, Law, Sex, and Christian Society in Medieval Europe, (Chicago and London, 1990).
Karen Cherewatuk, Marriage, Adultery and Inheritance in Malory's Morte Darthur, (Cambridge, 2006).
David Crouch, The Normans: The History of a Dynasty, (London, 2002).
(eds) I. Davis, M. Müller, and S.R. Jones, Love, Marriage, and Family Ties in the Later Middle Ages, (Turnhout, 2003).
Susan M. Johns, Noblewomen, Aristocracy and Power in the Twelfth-Century Anglo-Norman Realm (Manchester, 2003).
Conor McCarthy, Marriage in Medieval England: Law, Literature and Practice, (Woodbridge, 2004).