In the 70's I witnessed, aside from stories I heard, two women that married fellow women in my town.
CONDITIONS FOR OCCURRENCE
TO PRESERVE INHERITANCE
An only girl-child who was traditionally licensed to procreate in her father's house ( Nhachi) without marriage and who has been unable to sire a male child or born as many male children as desired can marry a fellow woman of her choice to help her populate her compound.
Once the bride price is paid, the bride follows her female "husband" home. The wife henceforth gives the female husband all the respects due to a husband.
It is the duty of the female husband to prequalify and choose the man or men who shall be sleeping with her wife for procreation purposes.
Her choice of a man depends on the characteristics she desires in her "children". She might go out of her way to beg a skilled man to reproduce himself in her house. Good men would always help. It is called "izelu agbo" meaning accessing and bring into a lineage some desired traits from an admired man.
Men who are chosen have records of success in their profession and are proven energetic persons. He could be a famous wrestler, successful farmer, wise and rich merchant or broad-shouldered, tall, gap-toothed handsome man.
The children from this arrangement are full members of the kindred (Umunna) meeting and are never discriminated against.
A SURVIVING WIDOW CAN MARRY A WIFE FOR HER DEAD HUSBAND OR SON
Some mothers and widows of men who died during Biafran war married wives for their relatives.
As usual, the bride price would be paid and a willing man asked to donate sperm manually to further the generation of the dead man.
In this case, the wife and the children answer the name of the dead husband/father.
BY STRONG FEMALE NARIVE DOCTORS
Many Igbo communities had very strong women medicine practitioners who by the virtue of their spiritual office couldn't marry or have sex.
They then marry a woman or women to have children for them.
A very popular one who specialized in exorcising mermaid spirits from ladies in my paternal grandmother's village married 3 women. Most of the offspring resemble the medicine woman's lucky brother.
The choice of sperm donors is for the female husband to make.
The concept of female-female marriage is not new. The difference is that todays's practitioners are now doing everything themselves without getting men involved for manual sperm donation.
There is nothing new under the sun.