Monogamy, may be due to what you have said, but it could also be due to the human need to be loved. I know id want someone around me who loved me, but why not 2 or more? Well, then jealousy would factor in, and in most situations cause alot of problems. In my opinion, monogomy is due to a deep connection that you will not likely find within another soul and therefore want to keep near to you, hence, marriage.
I asked this question to a friend, so you get another view, this is what she replied:
i think its both
but they're both almost as one
cos u need exclusive access to someone to feel secure, but u need to feel secure and feel like u have something and someone to get rid of jealousy of someone else
also because its not right to play around tho
Hope this helps