You have to understand that, for Marty, the circumstances involved in a moral decision count for nothing. Let's say that a woman leaves her husband because he beats her. They both remarry. In Marty's eyes, both are equally bad. 

Marty's statement "The Church ban on abortion does not hinge on whether or not the fetus is a "person", which is a legal term, or murder" shows, once again, his ignorance. The belief that the fetus is a person is EXACTLY why the magisterium says it is immoral.