prisons and Incest

Both the Incest and the Prisons are drastically changed from time to time and place to place. In some cultures incest is completely acceptable while in others its the worst sin/crime/act imaginable. Same goes for crime, in some cultures what is punishable by death is not wrong at all in others. This brings everything to the point of is it wrong because we say it's wrong or do we say it's wrong because it is actually wrong. Do we punish because someone breaks the law or do we punish because we believe what they did was wrong.
Scientists are trying to prove that incest causes babies to be disfigured and dimented so that they can say yes incest is wrong and this is why its wrong. What if there unable to prove it, or there findings and disproved, what then? If we cant use science to prove incest is wrong do we accept it? Do we switch from science to religion to help prove our thoughts? Lots of cultures are totally fine with incest and even view it as "pure" is it society that views incest and murder as wrong or is it actually wrong.
Once the question is asked one must decide how to answer it. Is it enough to use people opinions as law or do we need something more. Must incest be proven unnatural in some sort of way to be considered wrong or is society viewing is as wrong enough to stabalize this thought. Same goes for crime, should crimes need to be proven wrong in some way to be illegal? Murder is taking a life, it is unfair and basically is easy to prove it's wrongness, but how could we prove the wrongness of smoking weed? It's said to be safer than legal drugs like alcohol. Should it be scientifically proven to be more harmful than other legal substances before one illegalizes it? Basically the question is why do some societies view things as right when others view them as wrong?