If situations are looked at from an economics viewpoint, they can lead to apparently paradoxical conclusions, as exemplified by the title. In this case, examining the setup in more detail resolves the paradox: more sex for the mainly abstemious, and hence probably uninfected, results in safer sex for their partners, who might otherwise be having sex with an infected person. And this slows the overall spread of infection. That is, someone else (here the rest of society) accrues the benefit.
Landsburg has two main themes running through his examples: how to provide the right incentives when different people bear the cost and reap the benefit, and how to ensure that all the costs and benefits are taken into account in making decisions. There are some fun examples, and some peculiar examples, including why you should contribute to only a single charity, why shopping trolleys are getting bigger, and why all the petrol stations suddenly putting up their prices together proves they aren't in collusion. There's the bit about the fact the population is getting steadily and significantly richer, and that our descendents will be (in today's terms) millionaires pretty soon. Provided this trend continues (and he argues that it will), it means that if you believe in Social Security (taking from the rich and giving to the poor), then rationally you shouldn't believe in conservation, which takes from the poor (us!) and gives to the rich (our descendents). Hmmm.
There appears to be the odd inconsistency in places. For example, Landsburg suggests that pension rates should be set by the young rather than the old, because the old only accrue the benefits, whereas the young not only accrue the benefits (if later), but they also bear the cost (now). But surely this difference in time between bearing cost and accruing benefit is crucial? He doesn't say so during the pensions discussion, but there is a later chapter on the rationale behind locking the fridge to prevent snacking (if you don't want to snack, why snack? If you do, why lock the fridge?), where he explicitly brings in the idea of negotiating with your future self as if that is a different person. However, most of the time he himself raises the "obvious" objections to his reasoning, and demonstrates why those objections fail to hold water.
Landsburg is a dyed-in-the-wool free marketeer, but he does end up with a section on "things that make him squirm", that is, problems that when analysed from an economics viewpoint suggest that government or other centralised intervention could produce a better solution. The various arguments throughout also help expose the underlying assumptions (for example, that the More Sex of the title wouldn't result in a major change in social patterns), helping demonstrate that in reality, things are a lot more complicated than some of these analyses might suggest. So, lots to argue with here, and lots of food for thought, and lots of fun to read (even if you are left spluttering much of the time).