The question relates to real situation where a child is actually raised by two adults. Irrespective of their gender.
Everyone agrees that ideally a child should be raised by both birth parents.
Reality means that the best interest of the child is best served by real situations, and sometimes that best interest means being raised by two adults (which is better than just one adult) who are not the birth parents.
So in these real situations, what is wrong with both “acting” parents having shared parental authority?