Can't read article but solving this theoretical (and perhaps now real) puzzle is the primary story in the relatively poor book "More Sex is Safer Sex: The Unconventional Wisdom of Economics" which came out in 2008. Don't bother with the book, it's essentially a lamer freakonomics, but it's interesting that the central story has come up.

Basically if you have many to many pairing up (many one person with one or two partners), there may be less mixing, and less transmission.

In a one to many pairing (one person with ten partners) you can more easily transmit disease even if the total number of pairings is fewer.