It's like the Muslim situation. Of course we know that not all Muslims are terrorist extremist, but many people believe they all are. The same can be said of all religions, walks of life, political views, and anything and everything else. Yes some priest did molest children. Does it mean all priest are child molesters? Of course not. One guy even claimed the Megadeth song "A Tout le Monde" inspired him to shoot up his school. It doesn't all metalheads are violent, it just means one person took something the wrong way and done something horrible. And saying their teachings are flawed because it says one thing and they do another does not reflect the religion, but the person.
And as far as "sin" if you look into it you can easily trace down why most of what is sin is sin. Not all of it, but a good number of it. Such as the dietary laws. Animals that are prone to making people sick, such as pork and shellfish, are forbidden to eat. People are getting sick more often from these animals, so it must mean God doesn't want us eating them. This is a very simple and easy to understand primitive perspective, no? And if you look at circumcision, cultures in desert environments largely practice it, so of course we should find the same thing happening in an area where it is common. As for adultery, when you view the bond of marriage between two people as sacred, and sex as something that goes along with it that isn't too hard to understand either, especially considering people then had very little to no knowledge of psychology, genetics, natural drives, and so on.
