I'm mostly in agreement with this, but feel I must point out that from the perspective of social primate evolution the "sex only when it will result in offspring" paradigm is a perversion invented (or at least reinvented) by modern humans. Sex is primarily a bonding mechanism, as evidenced by the fact that sexual desire is mediated as much by social circumstances as by other considerations. Of course, social standing is ultimately directed at improving genetic fitness, but sex has been repurposed by the primate social system so that, essentially, it improves fitness in two ways rather than just the one you seem to be seeing. Given this, and the fact that the important number is (as some evolutionary biologists have pointed out) not the number of children born but the number of one's children who themselves reproduce, and you have a perfectly good reason why humans in every place and time have been trying like he'll to invent reliable birth control, for those numerous times when the "social bonding" part is desired but not the "potentially getting pregnant" part.
This is extremely interesting to me because I am such a person; I have had significant difficulty throughout my life with uderstanding the existing state of other people. I've luckily found a mate who is much better at it than I am, and can therefore pull me aside if necessary to tip me off that I'm talking at cross purposes with my interlocutor. However, this is my own problem to solve.
What I want to know, though, is this: Is "a single step" of a particular reliable size, or do people take differently sized steps?