The obvious answer to Tyler's puzzle about why people don't have more sex is that the cost is not in fact low. It strikes me as bizarre on its face to think of sex as a low-cost activity. Most people don't want sex, per se, but want sex with a person with whom they want to have sex that wants to have sex with them. For many, then, supply is low, and search costs are high.
Even within the context of a relationship, there may be sticky emotional issues that raise the cost. There may be performance or aesthetic anxiety. I think many partners in effect trade for their preferred kind of sex, but the price needs to be right. A does X to B and B does Y to A. However, although A would love him some Y, he finds doing X boring, and B is craving a bit of X, but Y-ing A sounds like a big drag. And so sex happens only when A wants it enough to not mind doing X for it, and B wants also it enough to not mind doing Y for it. If A gains ten pounds, and B has been acting like a jerk lately, it just might not happen at all. And the expected cost of getting sex outside the relationship seems even higher. So we get investment in poor, low-cost replacements, like pulsating showerheads and streaming video.
[Update: And I should have mentioned religious guilt. That's a real cost for many, many people.]