Why have a total ban on inbreeding in humans when it is basically 50/50 between pro and con doing it?

Going on a pure science progression sans God what would or should stop selected inbreeding? In many times over human existence it has occurred, mostly on the 1st cousin level, but has nonetheless. It happen in Amish communities because they were isolated and not really a large number of them at certain places and times, with the British royals of old to keep power and status in the royal family, in Japan after the A-bombs were dropped and population around ground zero was very shallow. Just as it can be used to eradicate certain diseases from a blood line or accentuate desirable traits it has the potential to accelerate negative traits; seeing there are no 3 armed Amish or Japanese with flipper feet it is pretty much 50/50. So if there is no sin in it, remember taking in account no God for this question, outside of individual personal ”ick factor” what reason would there be for a total ban on it?