Or was the need to secure titles and lands considered more important than increasing degradation of their family?
Obviously they didn't know about genetics. but this has been a somewhat common practice and did the families realise that the ongoing increase and severity of deformity and disease was the result of intermarriage?
You didn't give any clear timeframe, but Queen Victoria certainly had some strong opinions on the matter.
"In1869 she had argued that it might be better to ' infus[e] new and healthy blood into it [the royal family], whereas all the princes abroad are related to one another; and while I could continue these Foreign Alliances with several members of the family, I feel sure that new blood would strengthen the throne morally as well as physically.'"
She also wrote in 1885, concerning the propsed marriage of her granddaughter - Victoria Moretta - to Alexander of Battenberg in 1885 'if no fresh blood is infused occasionally the races would degenerate physically and morally'
Source: The War of the World, Niall Ferguson, p96
Followup question: What was the papal opinion on the topic? Were such familial arrangements discouraged?
You're going to need to give us a "when" to go with the "what". Are we talking Victorian hemophiliacs, or are we thinking more Hapsburg lips?
fyi, you'll find some previous discussions here
Followup question: Why didn't European royal families intermarry with mongol, middle eastern, asian, and other royal families?
So, can you cite some examples of progressively worsening problems?
A lot of people are saying that inbreeding actually carried a small or even negligible risk in defects. But are there ANY other possible physical/mental risks that can incur through inbreeding? Or, is it really possible to have generations of cousin marriages or uncle/niece marriages and have the descendants no less healthier than the average population?