Found the below text. Although this is a huge wall of text, I think it provides good context to what we are discussing about. He is talking about marriage between cousins for the most part here. I also remember reading in another chapter that the most common form of incest (as in sex within same family) was mostly between a father and a daughter. This is because people do not develop sexual feelings for people they grew up with as kids and a father could be sufficiently distanced from a daughter's life for this to happen. I believe this was discussed in Chapter 8 or 9 in the book.

Let me put in a disclaimer that I do not own the copyright for the below text and I will remove it if need be. That said, here is the text from Chapter 7.

But at the top of society, the opposite prejudice prevailed. Medieval lords banished many of their daughters to nunneries. Throughout the world rich men have always favored their sons and often just one of them. A wealthy or powerful father, by leaving his status or the means to achieve it to his sons, is leaving them the wherewithal to become successful adulterers with many bastard sons. No such advantage could accrue to wealthy daughters. This has a curious consequence: It means that the most successful thing a man or a woman can do is beget a legitimate heir to a wealthy man. Logic such as this suggests that philanderers should not be indiscriminate: They should seduce the women with the best genes and also the women with the best husbands, who therefore have the potential to produce the most prolific sons.

In medieval times this was raised to an art. The cuckolding of heiresses and the wives of great lords was considered the highest form of courtly love. Jousting was little more than a way for potential philanderers to impress great ladies. As Erasmus Darwin put it: Contending boars with tusks enamel'd strike, And guard with shoulder shield the blow oblique; While female bands attend in mute surprise, And view the victor with admiring eyes: So Knight on Knight, recorded in romance, Urged the proud steed, and couch'd the extended lance; He, whose dread prowess with resistless force, Bless'd, as the golden guerdon of his toils, Bow'd to the Beauty, and receiv'd her smiles."

At a time when the legitimate eldest son of a great lord would inherit not only his father's wealth but also his polygamy, the cuckolding of such lords was sport indeed: Tristan expected to inherit the kingdom of his uncle, King Mark, in Cornwall. While in Ireland he ignored the attentions of the beautiful Isolde until she was summoned by King Mark to be his wife. Panic-struck at the thought of losing his inheritance but determined to save it at least for his son, he suddenly took an enormous interest in Isolde. Or at least so Laura Betzig retells the old story: Betzig' s analysis of medieval history includes the idea that the begetting of wealthy heirs was the principal cause of Churchstate controversies: A series of connected events occurred in the tenth century or thereabouts: The power of kings declined and the power of local feudal lords increased. As a consequence, noblemen gradually became more concerned with producing legitimate heirs to succeed to their titles, as the seigneurial system of primogeniture was established. They divorced barren wives and left all to the firstborn son. Meanwhile, resurgent Christianity conquered its rivals to become the dominant religion of northern Europe: The early Church was obsessively interested in matters of marriage, divorce, polygamy, adultery, and incest. Moreover, in the tenth century the Church began to recruit its monks and priests from among the aristocracy.

The Church's obsessions with sexual matters were very different from St.Paul's. It had little to say about polygamy or the begetting of many bastards, although both were commonplace and against doctrine. Instead, it concentrated on three things: first, divorce, remarriage, and adoption; second, wet nursing, and sex during periods when the liturgy demanded abstinence; and third, " incest " between people married to within seven canonical degrees. In all three cases the Church seems to have been trying to prevent lords from siring legitimate heirs. If a man obeyed the doctrines of the Church in the year 1100, he could not divorce a barren wife, he certainly could not remarry while she lived, and he could not adopt an heir. His wife could not give her baby daughter to a wet nurse and be ready to bear another in the hope of its being a son, and he could not make love to his wife " for three weeks at Easter, four weeks at Christmas, and one to seven weeks at Pentecost; plus Sundays, Wednesdays, Fridays, and Saturdays—days for penance or sermons; plus miscellaneous feast days." He also could not bear a legitimate heir by any woman closer than a seventh cousin—which excluded most noble women within three hundred miles.

It all adds up to a sustained attack by the Church on the siring of heirs, and " it was not until the Church started to fill up with the younger brothers of men of state that the struggle over inheritance—over marriage—between them began. " Individuals in the Church (disinherited younger sons) were manipulating sexual mores to increase the Church's own wealth or even regain property and titles for themselves. Henry VIII's dissolution of the monasteries, following his break with Rome, which followed Rome's disapproval of his divorcing the sonless Catherine of Aragon, is a sort of parable for the whole history of Church-state relations. Indeed, the Church-state controversy was just one of many historical instances of wealth-concentration disputes. The practice of primogeniture was a good way to keep wealth—and its polygamy potential—intact through the generations. But there were other ways, too. First among them was marriage itself. Marrying an heiress was always the quickest way to wealth. Of course, strategic marriage and primogeniture work against each other. If women inherit no wealth, then there is nothing to be gained from marrying a rich man' s daughter. Among the royal dynasties of Europe, though, in most of which women could inherit thrones (in default of male heirs), eligible marriages were often possible: Eleanor of Aquitaine brought Britain's kings a large chunk of France. The War of the Spanish Succession was fought solely to prevent a French king from inheriting the throne of Spain as the result of a strategic marriage. Right down to the Edwardian practice of English aristocrats marrying the daughters of American robber barons, the alliances of great families have been a force to concentrate wealth. Another way, practiced commonly among slave-owning dynasties in the American South, was to keep marriage within the family. Nancy Wilmsen Thornhill of the University of New Mexico has shown how in such families more often than not men married their first cousins. By tracing the genealogies of four southern families, she found that fully half of all marriages involved kin or sister exchange (two brothers marrying two sisters). By contrast, in northern families at the same time, only 6 percent of marriages involved kin. What makes this result especially intriguing is that Thornhill had predicted it before she found it. Wealth concentration works better for land, whose value depends on its scarcity, than for business fortunes, which are made and lost in many families in parallel."

Quite right and in many cases, and it is still the case in some parts of the world, having a family also ensured that you had someone to take care of you as you got older. I think one of the reasons that we are seeing so many issues here in the west is that we have too many safety nets.

Women don't have to feel the pinch of their bad decisions because daddy government is there to catch them when they fall.