Question: Nature vs. Nurture

So since I didn't feel like writing anything today, I have a question I wanted to revisit.

A while ago me and damn near all of my facebook friends were having an argument on whether or not it was NATURAL to be monogamous. It's my personal opinion that it's not NATURAL to be with one person sexually, but societal norm natures us to believe that this is how we should be...

I need to know though, once and for all. Is it natural to be with just ONE person? And if no, then can you blame your partner for cheating on you??