The case outlining that a step-parent has no legal obligation to support the child is In re Marriage of Carney, 206 N.W.2d 107 (Iowa 1973). Wife had a child from a previous relationship, and while Husband had started the step-parent adoption, it had not been completed when the couple decided to divorce. Husband asserted he intended to always support the child.
However, the Court did not enter an Order requiring Husband to support the child because it did not believe it had the power to do so. The only individuals who could be forced to pay support were "parents," which was defined as a natural or adoptive parent only. Individuals standing "in loco parentis," or one who put him or herself in the position of a parent by assuming the obligations of parenthood, without the formalities of adoption, could not be forced to pay support. Without the formalities of adoption, Husband was entitled to disavow his voluntary responsibility toward the child. Further, because the child was not a product of the marriage, the Court did not have jurisdiction over the child in the divorce, meaning the rights and obligations toward the child were not issues the Court could consider.
A later case attempted to provide some rights for a step-parent, but again failed. In Petition of Ash, 507 N.W.2d 400 (Iowa 1993), Husband tried to use what other states have called the "equitable parent doctrine." This had been used other places to allow those who had acted as parent for a number of years, even if the formality of adoption hadn't been completed, to stay involved in a step-child's life after divorce. The Iowa courts did not extend this doctrine, however.
The bottom line: if you are an ex-step-parent to a child, that is all you are. No visitation, no support. You are a legal stranger to the child.