Why don't people have more sex?

Loyal MR reader Michael Vassar writes:

...all forms of consequentialism have a great deal of difficulty interpreting sexual behavior.  To put things short, there is an inexplicable shortage of sex.  Given that studies show that women and men enjoy it more than most other activities (on average, not on the margin I'll grant), and given its intrinsically low cost, it appears that even a crude approximation of a utility maximizing person would probably spend much more time having sex than most do. Do you know of any economic discussion of this?

We need not just reasons, but rather gains-from-trade-defying reasons.  I can think of a few:

1. The long-run lifestyle costs of being "more open to sex" involve a loss of integrity and control.  (OK, but I know many married couples, not all of whom hate each other, who don't seem to have much sex.)

2. 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?

3. Freud was right and we are all repressed.  The will is not unitary and the utility-maximizing part is not always in control.

4. There has been a market failure, but the Internet is remedying it.  People are having more sex and this will only go up.

5. Sex stops being fun when you do it to close a gap between your marginal utilities.  It requires spontaneity or some other quality inconsistent with the classical model of the consumer and the equation of marginal rates of substitution.

6. Sex isn't as much fun as the studies indicate.  Perhaps people lie about their quality of their sex or remember only the better experiences.

7. People want their sex to consist of peaks, rather than seeking to maximize lifetime utility.  Tom Schelling once told me this is why he did not listen to Bach more.

8. The market-clearing price for more sex is positive, and people feel shame about paying too explicitly; see also #5.

9. We are biologically programmed to "stick to our guns," rather than just kiss and make up; read more here.  Or perhaps your wife has read my earlier post on why it is hard to avoid torture.

10. People are having sex in other ways.  Maybe that is really good too.

11. Everyone else is having sex all the time -- Michael Vassar simply doesn't know about it.

12. You're all addicted to reading blogs.

My wife's question: "Should you be flattered or insulted that you are considered an expert on this?"