I can’t help but suspect that this will be a non-issue in a couple hundred years… speaking of which, marriage as we know it is a relatively new thing; I believe it really started in the middle ages.
This is incomplete. There were church sanctioned weddings even earlier, but these were more a recognition of something that was often accomplished as the result of a couple deciding that they were married in the eyes of God; the ceremony was something of a community wide acknowledgment.
Complicating the matter is that the Church sometime in the early Dark Ages adopted the current rule of consanguinity to prevent first-cousin marriages. The reason was to prevent the creation of excessively close clan systems, such as is common in Iraq–and for very similar reasons. In England, the reading of the banns in church for three consecutive Sundays in both bride and groom’s parish churches was to make sure that if anyone knew that the bride and groom were more closely related than they supposed (as was quite possible in communities where more than few people were born on the wrong side of the sheets), it could be quietly communicated to the priest before the marriage was formally recognized.
