Right so straight off he is assuming cousin marriage was the norm before the church stopped it. This isn't the case, it differed from culture to culture even within Western Europe. The main driver of cousin marriage is indeed passing on property to your extended family but that's also which makes the practice variable. During the majority of human history most people haven't owned any property to pass on. In Western Europe most people were serfs, slaves tied to the land, and even after they were emancipated in the British Isles most people were tenant farmers rather than owning land. So the church banning cousin marriage had zero influence on most people's way of thinking. If anything the West's weirdness comes from the fact people had miserable lives with no reason to commit to or care about a community which they could be turfed out of at any moment.