For me, its secondary, perhaps tertiary, unless you want to get all Freudian on me.
I'd argue that the reason people behave the way they do is because society, especially the media, conditions them to be that way (such as guys seeking sex without regard for anything else). Their motive, of course, is actually to feel effective more so than it is to have sex for the sake of having sex. Ever heard a bunch of guys talk about the girls they sleep with? No, because you're a girl, no offense. Some guys will lie about having slept with someone because having slept with someone is a sign of efficacy, for whatever stupid reason (I blame the media). We could certainly structure things differently, but the current structure is so effective for business that to fight it would be almost entirely futile politically, especially because people are mistaken into believing that it is "human nature" or something like that when really things are the way they are because we made them that way.
I actually think that sex is, in itself, unimportant compared to the need to feel effective. In our society, for men to have frivolous sex is to supposedly be effective while for women to keep letting these horrible guys sleep with them is to feel effective. This is incredibly powerful. Because of the way things are, many people have their efficacy tied to their sexual performance. I suppose it is natural to want to have sex, but things have been blown greatly out of proportion by the media.
