This is simply false, which point I’ve made more than once. I suppose it depends on the meaning of the term common. Only a tiny minority of the bourgeoisie engaged in first-cousin marriage. If you want to call that “common” then have at it. I’d call it misleading. And what’s with bringing up incest, in which almost no one in the civilized world engages?
Your use of writers as ‘evidence’ for your thesis points to the divide between science and the humanities: C. P. Snow’s Two Cultures. Or, in more contemporary parlance, the takeover by the Theater Kids — Justin Trudeau being the quintessential exemplar. It’s the triumph of emotivist ethics and feel-good over reality, and woke modernity is its endpoint. To quote a theater kid, “If the rule you followed brought you to this, of what use was the rule?”