Since we are talking of marriage among first cousins, I will share the Indian culture regarding first cousins..
The first cousins are of two types (My own terminology)..
1. Straight Cousins: They are father's brother's children, or mother's sister's children..These straight cousins are equal to one's own brothers and sisters. In fact no Indian language has any specific term for these type of cousins..They are simply referred to and addressed as brother and sister..
So marriage among staight cousins is out of question..
Example of straight cousins:
William/Harry/Beatrice/Eugenie/James/
Queen/DoK/DoG/Prince Michael..
Charles/Anne/Andrew/Edward/Sarah/Linley..
In these three sets, each person is a brother/sister of all other members of that set..
2. Cross Cousins: They are mother's brother's children, or father's sister's children..They ARE NOT equal to brother/sister, and there are specific terms for them.. Marriage may always be considered among these cousins..
Examples of cross cousins:
Peter and Zara are cross cousins of Will, Harry, Bea, Eu,Louise,James.
Prince Felipe is a cross cousin of Prince Pavlos
Earl Spencer's daughters are cross cousins of William and Harry..
The marriages among cross cousins were quite frequent till a few generations back.
A cross cousin of same age group was a potential alliance right from birth. In fact there were anxious grandparents waiting for their cross-grandchildren to grow up soon, so that they could marry them off..
But nowadays, no one's bothering that.. Cross-cousins are also being seen as "just cousins" and nothing more..
I wonder if this made any sense..