Here's a pretty important question: is it ok to have sex with many women provided that you end one affair before starting the next one? Is serial monogamy ok? - in which case the message is "Be faithful (to one woman at a time)".
Mathematical modelling suggested that HIV will spread particularly fast in communities where men typically have a wife and a mistress. This is said to be common in sub-Saharan Africa and so explain the very high rates of infection there.
But this paper, based in the real world, contradicts the model prediction: what matters is how many women a man has had sex with. Serial monogamy is not ok.
The actual method is complicated because the authors wanted to answer a geographical question: rates of concurrent and lifetime partnership vary from community to community and they tried to extrapolate from survey data, which only provides particular data points, rates across an entire area. This entails modelling, and geographical modelling gets complex very quickly. But the end result was that women are more likely to be infected with HIV in communities where the men have a lot of lifetime partners; communities where concurrent partnership is common does not raise the risk (except insofar as it implies a minimum of two lifetime partners).
