The incidence of birth defects among children of 1st cousins may be offset by a lower incidence of cancer. Some studies have indicated that diversity between mates increases cancer risk. This is thought to be due to a higher ratio of genetic mismatches. When two families have been separated for 1000 generations, then they have diverged to a greater degree than when the family has been separated for 1 or 2 generations.

An illustration of this exists in nature. When a lion male and female tiger are cross bred it produces an offspring known as a "liger". Ligers are larger than either parent and share characteristics of both. The problem is they almost always die of cancer. The thought is that there is too much genetic dissimilarity. That mismatch of genes provides a higher number of opportunities for cancer, due to cancer occurring when several genetic problems occur at once in the same cell.

If all of this is true, then that might explain the higher incidence of cancer in the US versus Europe, Japan, etc. since the US is more genetically diverse than most of Europe or Japan etc.

Getting back to marrying cousins; if you marry your cousin and produce children without birth defects, those offspring might be more resistant to certain cancers. So its pay me now or pay me later.