How many wives did David have? The only time God was angered with him was when he slept with someone else's wife. What about concubines? And since you bring up marrying children, in my grandmother's day the view was completely different on what a "child" was. She told me if a girl was sixteen and not married yet people started calling her an old maid. Most girls were married by age fourteen. In Bible times it was viewed differently too. I have heard historians and theologans say that Mary was probably about 13 when she married Joseph, and Joseph had already been married and was a widower...he was around the age of seventy. I don't defend these views in today's times, it is just interesting how our views change over time.
Just like drinking alcohol. Kids drink wine overseas and no one thinks anything about it. The same happened back in Bible times. Jesus did drink. I think most people started viewing drinking as a sin after the prohibition here. Excessive drinking and eating were always considered sins, but a drink of wine wasn't.
Well they also thought it was normal to marry your realtives...
Frankly I think Autism, Schizoprehnics, and other inherited diseases proably came about from marrying and having children with close family members.