Marriage is between two unrelated women

On one hand being the closeted pervert she was, Jane could not resist bringing her fetishes for and polygamy to life but on the other she couldn't bring herself to make people too inbred and insular to form a civilization. With a few minutes of thought she came up with the perfect compromise...

1: "All pregnancies produce twins. Usually sororal*, sometimes identical"

2: "Twins are incapable of reproducing together"

3: "All relationships between twins are romantic and become sexual on reaching maturity"

4: "While they are intimate, pleasurable and taken seriously twin relationships are not considered complete. For this joining with another pair is needed"

* Sisterly