http://www.aaronsw.com/weblog/savagesex

"..the spirits caused pregnancy, not sex.
They argued the case quite logically. After all, they noted, one fellow went on an expedition for a year or two and when he came back, he had a new son. He obviously wasn't having sex with her while he was away, so where did the kid come from? (Cough.) And, they note, there are some really hideous people on the island who nobody would dare have sex with, yet they manage to become pregnant. (Malinowski spies some kids looking sheepish when this subject is raised.)"