This objection to Marquis argument says that although the argument is valid, and although premises 1 and 2 may be true, the argument is unsound because premise 4 is false.
This is true because the abortion issue does not concern "just" the interests of the fetus.
It also concerns a person's basic moral right to control what happens in and to his or her body. This includes a woman's basic moral right to choose whether or not to continue a pregnancy.
So Marquis may be right that a woman has a prima facie obligation not to deprive her fetus of a future like ours, but even if this is correct that obligation is outweighed by her right to chose to end a pregnancy. So while it may be true that a woman has a prima facie moral duty not to end a pregnancy, it is false that in the vast majority of cases, there is no other moral consideration that outweighs that duty.
Therefore, premise 4 is not only false, but the entire argument of Rachel's, so it seems, is naive.
