There are two types of evolution. Genetic and memetic.
Genetic refers to the evolution of genes. Memetic refers to the evolution of ideas.
Things that exist in and are caused by nature are natural. Ideas come from our minds. Our minds come from nature. Therefore, they are both natural.
I believe it is natural to be monogamous and non-monogamous.
However, I believe we are genetically non-monogamous and memetically monogamous.
Photo Credit: Jana Knauerova for Wonderland