This is the actual explanation. With one caveat... slavery.
Basically, agriculture allowed people to specialize. This included specializing in killing. (Soldiers). You would use these specialists to conquer other lands. Which, in turn, provided slave labor to increase production in agriculture and other sectors. So the conquered females would be used as "rewards" for the conquerors. And the conquered males would be used to increase production across the conquering society via slave labor.
So the explanation is pretty simple. In fact, the 17 to 1 figure is so low that I question whether they aren't missing something ??? Societies that developed later generally required enormous amounts of slave labor. That, coupled with the fact that there would have been an enormous amount of killing involved in taking the slaves in the first place, makes me wonder how the numbers turned out so evenhanded? I would have expected it to be an order of magnitude higher. But they did the study and have the data... so life must not have been quite as brutal as I thought it was.
I suppose it is possible that life was brutal... and less desirable females simply may not have been taken? This happened a lot in our own history. Slave auction records show that some female slaves in America sold for as much as 100 times the rate of a "regular" female slave. (And even more in some private trades we have records for). We can speculate on why that may have been... but I think it's safe to assume that in a less sophisticated time they may have simply dispensed with females and males who were in some way undesirable, or were not likely to survive the journey back.