Why do you think that human cultures have laws against marriage between close relatives? On the average, each human has about six recessive...

Any individual who inherits two copies of the same lethal recessive allele dies. 

The odds of two unrelated individuals who decide to have kids both carrying the same type of lethal recessive allele are relatively low.  However, since close relatives share a high percentage of their DNA, it is highly likely that they will carry the same type of lethal recessive allele.  So, if close relatives have children, there is a much higher chance that the kids will inherit two copies of the lethal recessive allele.  So, really, the law is designed to prevent reproduction between close relatives, but marriage is much easier to control than reproduction.

Even in cases where the offspring avoid a lethal recessive condition, there is still a high probability that they will inherit other problems/defects due to the close genetic makeup of their parents.