I have to admit this intrigued me and had to see if it worked. Boy did it work.
I made a male couple, one of them had the nurturing trait and when the couple wished for a baby I took that one into CAS and just switched to female. I have to tell you she turned out gorgeous.
The first time they had triplet girls, the second time twin boys. Once the boys were born I took the mother back into CAS and turned her back to him.
When you look at the family relationships both adults are listed as father.