What could cause a humanoid race, or even a group of humans, to develop a shorter sexual refractory period (that is, the males can have sex again more quickly after an orgasm)?
Many male mammals with a short refactory period are dealing with only infrequent periods of female reproductivity, i.e. they are "in heat" so conception MUST occur in a specific period. Thus those male mammals have to supply a lot of sperm into as many females as possible within a few days. Human females have a monthly fertile period that can't be easily predicted, so there is no need for a 1-2 day period of near constant sex, just 1-2 bouts of intercourse a day suffices to ensure conception within a year.
So you would need to drive human females into a less frequent fertile period (like just once or twice a year) and possibly have multiple females in close proximity synch their fertile periods (this supposedly already happens to some extent in close knit female groups) such that a single male would have to engage in extensive bouts of intercourse with possibly multiple females during this brief period of fertility. This would favor males with shorter refractory periods would could impregnate multiple women (a king with his harem, essentially). Note that male mammals that deal with this undergo an "in heat" process themselves versus their normal behavior. I suppose one could argue that human males are constantly "in rut", at least in their younger years :)