Both Mary and Bob created the sculpture. Since there are no other things in play, it remains the property of them both.
What about Mary's choice of whether to destroy the ice sculpture or not? I don't see where this analogy breaks down.
For a child, at the act of conception, both have a certain 'right/responsibility' to the child. After conception however, Mary is given the choice to abort the pregnancy, which completely overrides any claim that Bob might have to the potential child. Bob could be completely and totally against the abortion, but that doesn't matter in the slightest.
For the ice sculpture, there is never a point later in the process where Mary is given sole control over the sculpture, because it never threatens her well being. As such, it remains the property of both.
On a side note, I don't think that 'creator' is the best word to be using, as who exactly counts as a creator is really rather subjective. I think it would be better to talk about the 'right/responsibility' of each party involved, as this can be more easily traced to their participation in various decisions in the process.
Basically, I think we should be able to answer this question:
Person A is involved in a pregnancy but doesn't wish to support a child. Do they have to?
without having to ask any additional questions about the gender of person A.