In the past why did God allow marriage between family members?

The first humans were perfect. Had they not disobeyed YHWH, they would still be alive today. Deformities, decay and medical anomalies have increased exponentially since the day that the first humans disobeyed YHWH. Initially the corruption to the genetic code of man was very tiny, that is why Adam lived so long. The genetic deformity increased exponentially with every subsequent generation until a time when severe deformity could occur to the genetic code of the offspring of a couple who were closely related. Thus, when that time occurred, YHWH commanded that mankind could no longer choose a marriage mate from a close relative.