Why don't people have more sex?

An older but still amusing post from Marginal Revolution.

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.