Western liberalism was built on the principle of marrying out. Our beliefs about the freedom of the individual ultimately stem from the Catholic Church’s ban on cousin marriage, which helped create a worldview that was open, trusting and opposed to both clannishness and xenophobia.

The medieval Church’s insistence that marriage be consensual was revolutionary and strange; back in the 13th century a romantic poem, The History of William Marshal, has the protagonist coming across an eloping couple who have defied their parents to seek true love.