> Opposition to abortion in the US is entirely premised on Christian ideology and the belief that a human is ensouled by God at the moment of conception, thus making abortion at any stage equivalent to murder

‘Entirely’ is a strong word: the existence of a single non-Christian American opposed to abortion refutes your contention.

Anyway, abortion isn’t (necessarily) murder: it’s homicide. Homicide can be legal, e.g. in the cases of capital punishment and self-defense.

> scientific consensus is that abortion is a perfectly valid and sometimes necessary medical procedure, not fundamentally different than removing an appendix

I think that you are confused here. The universal scientific consensus is that neither a blastocyst nor an embryo nor a fœtus is an organ of his mother, but a separate organism. That is, there is a fundamental scientific difference between removing an organ from a human being and removing a human being from another human being.

> Science certainly doesn't support outlawing abortion outright, or banning contraceptives or forcing women to carry nonviable pregnancies to term

‘Science’ doesn’t support any particular policy decision all, but only deals in facts; it doesn’t support outlawing abortion and it doesn’t support mandating it, and likewise with anything else. All science can do is provide estimates of facts, and predictive estimates of outcomes: it can’t say whether those outcomes are good or not, because good and evil are not intelligible to science.

I suspect that you think they are, because you assume that whatever makes the most people the happiest is the best outcome, but that’s a moral call, not scientific: someone who wanted to make one single human being the happiest possible could just as easily use scientific means to achieve that goal instead.

I would not even agree with your contention that science says that abortion is a necessary procedure, because it begs the question of what ‘necessary’ means. Science says if that an ectopic pregnancy is not aborted, the mother will almost certainly die; some (most? all?) moralities say that it’s wrong to prevent taking action to prevent that outcome — and probably a few say otherwise. Certainly it cannot address ‘validity,’ because again that doesn’t even make sense. Science says if one achieves a certain physical arrangement of items in a certain condition then one can release a tremendous amount of energy; it can predict what the effects will be; it can’t say anything whatsoever about whether or not bombing Hiroshima is ‘valid.’