Incest- why cannot we sleep with our brother?
Why can you not rob a bank? Or kill your grandmother? Why can a man not have many wives? Or marry a 11 year old girl?
Answer: Because it is against the law, approved by the majority.
Want to argue with law vs moral code? Go ahead. I have had kids and parents argue about chewing bubble gum in class at school, but it was to no avail. The majority do now like to step on bubble gum on school floors, or find it under their desktop. It turns out, in this world, the majority establishes the norms for society.