Deep in the mountain is a truly easy place. Far from human, no predator and abundant food.
The clan living here is unique, because it consisted of only Reimu types. The place is so safe and plentyful that even the commonly regarded as useless Reimu types can continue their life.
Of course, they marry other Reimu. It's not like they have any other choice. Resulting children are of course, all Reimu. The Reimu usually switch role of being father and mother.
One day, a Reimu is giving birth. A lot of other clan member is present because it's a rare mammalian birth.
The baby is a changeling, a golden haired, black hatted Marisa.
Being unique, Marisa is loved by the clan. Not just children her age, but also the adults. The only yukkuri that doesn't want to marry Marisa are her parents.
Time passes, finally the juvenile Marisa left her home. Feeling lonely, her parents make another little ones. This time, six fruits hung from the stalk growing from Marisa's father (now taking mother role).
Outside, Marisa is met by Reimu who had just divorced from her mate to marry Marisa. Of course her mate agree, both wanted to marry Marisa.
Not just them, their children, and all other Reimu in the clan wanted to marry Marisa. Even pregnant mothers would break their stalk on their own.
Confused, Marisa made a decision. She loved everyone and while she wont marry, Marisa will refresh with any Reimu who wants it.
The orgy begin and younger yukkuri all died because they're not ready to bear child. Everyone is too consumed in lust to care though.
Finally, Marisa died from losing too much paste as sperm. The yet unsatisfied Reimu look for release in each others and finally they too died from exhaustion.
Marisa's parents woke up and went for a walk together. They're shocked to find the clan members all died.
But they have the responsibility to go on with life. Reimu had six children now, so they must take it easy for everyone who has died today.
The six children married among their own and starts populizing the forest again.
This is a natural system. When the number of Reimu in the clan reached a critical limit, a single Marisa will be born and the clan will meet the same end as today.