Mientras mås prima, mås arima.
The more a cousin, the closer one gets.
The saying actually uses Spanish words, but our mañaina understood them perfectly well. The proverb cautions against the assumption that, just because the boy and girl are relatives, the two of them will never develop feelings for each other. Close blood lines do not always prevent romance. So, the proverb teaches, keep your eyes on them! Danger, danger!
In English, the phrase is "kissing cousins." Generally speaking, people of many cultures frown on first cousins becoming romantically linked. It's just too close for comfort. Then there is the belief that the children of parents who are first cousins have a greater chance of suffering mental deficiencies.
The Bible (Leviticus) talks about avoiding relations with close relatives. But this has generally been interpreted as forbidding marriage between siblings. Isaac and Rebecca are a well-known biblical example of marriage between first cousins once-removed.
In Catholic Church law, first cousins cannot marry unless the permission of the local bishop is first obtained. So there is no absolute restriction in the Church against cousin marriages, and some of our mañaina did marry their first cousins, but it was always looked on with some discomfort by most people.
In the United States, it depends on what state you live in. In Massachusetts, you can marry your first cousin. Drive a few miles into neighboring New Hampshire, and you cannot. In some states, you can marry your first cousin but only if you are too old to bear children or otherwise cannot have them. All states allow the marriage of second cousins.
Albert Einstein married his first cousin, and so did Queen Victoria of England. Charles Darwin married his first cousin, too.
As for traditional Chamorros, it doesn't matter that such illustrious people married their first cousins. For many of our mañaina, even marrying a 2nd or 3rd cousin is taboo.
For this reason, in the past, grandparents always asked the new girlfriend or new boyfriend, "Håye tatå-mo? Håye nanå-mo?" "Who is your dad? Your mom?" just to make sure they weren't 2nd or 3rd cousins.