Suppose mom has a child by another man and, under presumption of paternity laws, the husband is listed as the father. Now suppose the man has a child by another woman. Is his wife now going to be listed as the mother on the birth certificate under this ruling? 

When Arnold Schwarzenegger had a child by another woman, that woman's husband was listed as the father. If equal is equal, under this new ruling, Maria Shriver would now be listed as the mother--because she was married to the father. 

Presumptive fatherhood at one time made sense. So, too, presumptive motherhood makes sense for two married women. I'm not sure it makes sense for a woman married to a man, if she could end up being the legal mother of a child she did not bear nor adopt.