i think the most important thing to remember about sex is that it produces quite a bit of pleasure and not much else. Nothing against sex but it just does not lead to bonding. Humans are not a pair bonded species - we lack the hormonal adaptations, brain chemistry and brain structure to fit into that category. None of the higher primates are 'pair bonded.' There are a few lower primates that do pair bond but not us. Many animals including humans will temporarily 'bond' in the pursuit of sex.
At the 2008 Nobel conference at Gustavus Adolphus College, several anthropologists including Svante Paabo agreed that humans are not a pair bonded species and conjectured that 'friendship' was instead the motivating force behind the human community. I would beg to disagree and suggest that the mother/child bond was overwhelmingly the original force behind human evolution and cooperation, and that was where the deep bonds lay. As an example of my point, in early urban history, long after societies had achieved living in extremely close and modern conditions, there were still no laws controlling female sexual behavior and consequently no such institution as 'fatherhood,' thus no pair bonding at all. It would seem nature probably intended humans to have indiscriminate sex whenever the urge hit. All this would seem to shed some light on why human attempts at pair bonding fail so often both officially and unofficially.