In this particular system, the parameters matter, but not very much for the particular question the author was asking. The system is well-behaved; it doesn't bifurcate or go chaotic. Tweaking the parameters will change whether the final level of infections is 10% or 90%, but the overall conclusion that "sexual conservatives taking more partners increases total infections" remains valid.
It is more plausible that the number sexual partners follows a Pareto distribution. It is likewise plausible that the economist sought to flatten the Pareto curve, even if this may result in the area under the curve increasing somewhat. Would it work? Maybe not. But this counterexample just reflects badly upon us. Programmers are the new physicists of this era.
If you really want to take the economist up to task, you should find his paper, code his model and parameters exactly, see if you reproduce his hypothesis over a number of trial runs (for all we know, he may not have run a sim at all, given the NYTimes' shoddy reporting).
My model does have a number of flaws (a uniform distribution is still not very good; there's no gender distinction; the transmission rate per encounter is artificially high; people only stay with their partners for a random-yet-uniform amount of time) but I'm not sure to what extent these matter.
Further, one can think of intuitive reasons that the conservatives having more sex might help. e.g. it lowers the chance of an STD going from a high risk person to a high risk person (which would cause it to be spread way more after). I have no idea who that makes right, I'm just saying intuitively one can think of simple reasons it might go either way and so the details definitely do matter.
This agrees with your first figure:
About one in six Americans (17%) had the genital herpes virus -- called HSV-2 -- during 1999-2004. But that is down from the 21% rate seen in 1988-1994.
But then closes with this, which seems very real world relevant to this thought experiment/theory/whatever you want to call it:
As its name suggests, HSV-2 isn't the only herpes virus out there. HSV-1, the virus that causes cold sores, is much more common. As of 1999-2004, 57.7% of Americans carry the virus -- down slightly from the 62% HSV-1 infection rate seen in 1988-1994.
There's some bad news here: HSV-1 is causing more genital herpes than ever before. About 2% of people with HSV-1 infection -- but not HSV-2 -- have genital herpes.
"Our findings are consistent with previous reports that genital herpes caused by HSV-1 may be increasing in the United States, as in other developed countries," Xu and colleagues note.
The researchers warn that the herpes virus that causes cold sores may one day become a more important cause of genital herpes. One factor: The increase in teen oral sex that's helping stop HSV-2 spread may be increasing genital infections with HSV-1.
I mean seriously. Half the folks who often have sex will have AIDS?