Well let's start from the Islamic angle as so many people have mentioned it, the Prophet(saws) has clearly spoke of marrying outside your tribe and far and wide. Among this own marriages just one of his 13 wives was a cousin, and her name was Zaynab Bint Jahsh(ra). Yes his own daughter Fatima(ra) married Ali(ra), but remember his other daughters all married outside their family clan, two of which were to Usman(ra). The other factor is that the clans at the time of the Prophet(saws) seldom kept up the practice of cousin unions through each generation, rather most did not do it, some did.
Now it is permissible to marry your cousin, but not encouraged. It's also permissible to do many other things but not encouraged, it's not encouraged because it will eventually become bad for you. Now if you look at when Hazart Umar(ra) came across one tribe and he noticed that they were all physically weak, particularly their legs were very thin. He enquired further and found out that they married exclusively within, he then forbade them from doing so, and told them to seek partners from other tribes and nations. So here we have one of the commander of faithful actually forbidding something which is permissible in order to save the health of a tribe.
There have been numerous issues to do with generational cousin unions, medical and social, and people need to understand the problem. If your parents are cousins then you should not be marrying the offspring of their brothers and sisters, who are more than likely cousins as well.
My own family my parents are not related, my grandparents were not related, in fact my grandmother is ethnically different and of solid Central Asian stock. Among my father's brothers and sisters, his sister married outside, both his brothers did as well. Their children, the sister has four children, three daughters married outsiders, her son married a cousin. My youngest uncle, one daughter married a cousin, the three other girls all married outsiders. My brother is married to a girl he met as well, so not in the family. That's all of us. Amongst my extended family, out of 40 marriages, four are to cousins, the rest to outsiders.
As for the wider community I have been to over 30 weddings in the last five years and I have not come across a single cousin union. The practice is fast dying in the UK as young British Pakistani males and females meet each other at universities, work places, social network meets, speed dating and so on.