Marriage to a cousin or second cousin nowadays considered unacceptable, but it was by no means always - a long time marriages between relatives were the norm.
Direct ancestor of Homo Sapiens have been variously estimated from 700 to 10 thousand people - incestuous relationship in this case were inevitable. Subsequently, this type of marriages, including to preserve the "purity of blood" and to preserve the continuity of the family (mainly practiced know). The young heir, for example, the princely family, carefully chosen life partner, not to denigrate the family bond with the commoners, avoiding the so-called misalliance - a big difference between the estates and property status of the spouses.
In the United States before the Civil War, the marriage between cousins is considered normal, but then people's views began to change gradually, and the proportion of such unions has declined steadily. In the XX-th century around the enlightened Western world such marriage gradually disappeared, but in the Middle East is still considered valid.
In general, according to rough estimates, 80% of marriages in the history of mankind were made man and woman, between them in the second cousins or more closely related.