Amalgam

In Venezuela, there's a tribe with a ceremony regarding pregnancy. They believe that a child is not set in its conception, and need not only have one father. So, after the first conception, the woman will go around to the other men in the village, trying to craft the perfect child out of the possibilities. Of course, in civilized society, we know that that's silliness, and is simply a great excuse to go swinging.

Right?

Meet Jane Doe, a girl from aforementioned civilized society, who has in fact never heard of the Bari people of Venezuela. However, what the females of the Bari tribe do for protection and food, due to the possible paternity of all the males she accepts sexual advances from, Jane doesn't even know she can do yet. She can bind these sperm together, and create a single, uber-child.

Today's the day that she finds out.