I tried to post this as a comment to Maybe Genius' awesome post, but Blogger is being weird. Since this is a really important topic to me, I decided to post my response here. (Make sure you read her post first.)
Totally agree here. Our culture places far too much importance on monogamy and "faithfulness." Monogamy isn't for everyone. You wouldn't limit yourself to only one friend, so why should you limit yourself to one lover? Many people need variety. And it is possible to love one person while still enjoying the sexual/romantic company of others. When I get into a relationship with someone, I let them know upfront that they shouldn't expect "faithfulness." (Nor do I expect it from them.)
One thing I appreciated about "The Girl With the Dragon Tattoo" was the way it presented open relationships. The protagonists not only had sex with multiple partners, but they also cared about these individuals. (Then again, this book was published in Sweden, so maybe this isn't as much of a stigma over there.)
Another example that comes to mind is Nnedi Okorafor's "Who Fears Death." While it doesn't exactly endorse open relationships, I loved the way the "slut" of the group was also one of the bravest and most loyal. It also wasn't afraid to acknowledge that women actually enjoy sex.