Well, the words step-father and foster-father indicate a relation with a child and say nothing about the relationship of the parents toward each other sexually.
Foster-parents are in an arrangement with the state to care for children lacking parental involvement, whether through difficulty, neglect or abuse. Jesus suffered none of these issues as the divine Child of Mary. The term, foster-father, to me is associated with the government's child services. Also, most foster parents have conjugal relations with each other, so what doctrinal clarification is one perserving with this term?
Step-parents are in a relationship with children via adoption (if the child is an orphan) or via a relationship with one of the biological parents (that relationship usually, but NOT NECESSARILY, involves conjugal relations with the natural parent of the child). The term step-parent at least implies love; love for the spouses for each other (apart from any conjugal relations in most,not necessarily all, of these relationships) and love for the children. Step-father does not necessarily imply conjugal relations between Joseph and Mary. It describes a relationship between Jesus and his earthly dad.
To claim that foster-father preserves the perpetual virginity of the Theotokos is merely an opinion. If you feel more comfortable with it, then use it. But if you really want to be safe, why not refer to St. Joseph as Jesus' Guardian?