The teaching that circumstances have no bearing whatsoever on moral decisions does not originate with Marty. But it is still wrong. 

If I were to take a rifle, track a man down, shoot and kill him, this would be immoral. Yet, in combat, I have done exactly that, with the blessing of Church and state. The same act, two different sets of circumstances, two different views of the morality of the act.

That the fetus is a person and abortion is killing that person is the basis for Catholic teaching that abortion is immoral. Saying that "personhood is a legal concept" is just Marty showing his ignorance. "Personhood" is also a theological term. The 5th century philosopher/theologian Boethius defined person "as an individual substance of a rational nature." Aquinas, the the Summa, I, q 29 takes this definition and considers its meaning at length. I will not go into the argument John Locke made in opposition to Aquinas, because I don't want to get bogged down by a discussion of ontology.