There are situations in the law in which the Courts have already dealt with the issue of whether a non-biological parent could be a "Parent" as well. For instance, in Massachusetts there is a Grandparent Visitation Statute that provides for visitation for grandparents with their grandchildren if they have been significantly involved in the children's lives. Although this standard is strictly applied by the Courts, a win on a grandparent visitation case is essentially a recognition that a child's grandparent is acting as another parent in some capacity and it would benefit the child to continue that relationship.
Similarly, the Courts in Massachusetts have also recognized "de facto" parents in situations where another person (such as an aunt or uncle or step-parent) has been so involved in a child's upbringing that they have become indispensable in that child's life. The Courts have awarded "de facto" non-biological parents rights of custody or visitation (and sometimes obligations for child support) when it is found that that person has been such a large part of the child's life that it is in that child's best interest to maintain that relationship.
Within this framework, the Court has in the past recognized that third-parties can sometimes be indispensable in the raising of a child. Despite the complications that a third player in the custody/visitation arena can cause, the Courts have recognized in some situations it is more beneficial than cutting that person out of the child's life.
Similarly, this same logic is being applied in expanding how the court views parenting arrangements created by "non-traditional" family structures. As discussed at length in a recent Boston Globe Article, Johnny has two mommies – and four dads, this is becoming more common when lesbian or gay couples involve a third biological parent in the conception of the child.
Also discussed in the article is the opposition by some to the inclusion of any third or non-biological parents in parenting (despite the proven value of quality involvement by step-parents in many families). Of course, much of this opposition is just a thinly veiled opposition to any recognition of gay or lesbian rights as depicted in this news story video posted on the website of colleague Gabriel Cheong. As depicted in this news video and in this article, the law is struggling to catch up with how to categorize these different relationships despite the fact that we already have some precedent in how we deal with "de facto" parents.
In the news video the victim-Mother indicates that she was advised she didn't need to adopt her non-biological child because the law in Vermont recognized her parental rights under her marriage. Since all states don't recognize that marriage, though, it might have been more prudent for her also to adopt the child.
Even for parents in more unusual arrangements that involve three parents, some jurisdictions are now allowing third-parent adoptions.
The takeaway point from this struggle should be that until the law becomes clear in how it treats these relationships documenting your rights and agreements should be clear from the outset. No matter what your family structure is like, if it is "non-traditional" the law might be uncertain in how it treats you or require more "proof" of parental relationship. In order to protect your parental rights you should explore your options when everyone is "getting along".