Inbreeding leads to offspring with serious deficits resulting from a homozygous recessive/defective at many genetic loci/genes.
Work Step by Step
Marrying one's close relatives means that ones mate will likely have a very similar genetic makeup. This, in turn means that many genes in any offspring produced will be in a homozygous condition, including genes for which both parents have recessive/defective alleles. This will lead to many offspring with genetically-coded defects, offspring with a great struggle in life due to genetic/biological causes.