The average utility of sex is high but the marginal utilities are falling off a cliff. You just don't want any more. But how many people are at this margin?If people enjoy sex as much as they say, it seems like they should always have more sex if the added pleasure of one more sex act is greater than the added cost. That’s basic cost-benefit analysis, right? But as I’ve argued before, it’s not necessarily true.
The key point is that the value of all acts of sex, not just the marginal act, may decline as the number of acts increases. Here’s how I put it before (yes, I’m actually quoting myself): “As the frequency of sex acts increases, it’s not just that each act is less satisfying than the previous one, it’s that the satisfaction from each and every act may decline. For instance, a person who has sex twice a week may appreciate each instance more (because of greater anticipation, built-up sexual tension, and so on) than a person who has sex every day.” As a result, the marginal utility of an act of sex is lower than the utility derived from your most recent sex act.
Suppose Phyllis is contemplating an increase in her sex frequency from thrice to four times per week. She finds that the fourth sex act, considered in isolation, will generate net satisfaction: the pleasure gained will be greater than the opportunity cost of not watching TV or getting another hour of sleep. Nonetheless, she rationally chooses not to increase her sex frequency. Why? Because if she has sex four times a week, every act will be less satisfying than it would otherwise be. Once that reduction in satisfaction for the first three acts per week is subtracted from the new satisfaction attributable to the fourth, the benefits no longer justify the costs. CSI: Dubuque it is.