@kellirosej,
Humans were designed to have sex with each other, and they weren't created to pair for life. If humans were designed to be joined with one person and never have sex with anyone else, then this is what humans would do naturally.
I don't know how much of life you have lived, but this is not how humans work. There are humans who mate for life... but they themselves will tell you that this is very hard work, and if they are truly honest they will tell you that many of them regret it (of course there are a few exceptions). But look at few many humans actually live their entire life without seeking additional partners.
The number is very very low.
When I use the word God I mean the deity responsible for creating humans the way that they are. If God had wanted humans to be strictly monogamous, he could have created us that way.
If that was his intention, clearly he screwed he pretty bad.