Sorry about the top post response to Sarvis. I accidentally found his post on a drive-by, but it’s, er, dead now. There’s a pretty simple answer, a variant of which Thrasymachus posted once in a thread now gone. I’ll repost the gist. The key is viability.
Sci-fi thought experiment: Gregor contracts a disease which gets him lodged inside Sarvis for one year, utterly dependent on him for nutrition, oxygen, physical care, etc. Sarvis must double his food intake, crap twice as much, wear an oxygen mask at all times and severely restrict his movements and lifestyle to allow Gregor to survive this ordeal. Should Sarvis be made legally liable to do so (a higher standard than non-coercive moral pleading)?
Morally, it can be argued both ways, but there is no compelling case to hold Sarvis liable at pain of criminal prosecution. Framed this way, it ought to be clear that the pro-life case hinges on more than a conventional “right to life” (the kind conferred on biologically independent humans). It hinges on a “right to life supporting sustenance”, which is a stronger claim. Many people would (sadly and reluctantly maybe) allow Sarvis the right to cut the lifeline through which Gregor is sucking his blood and since the resultant corpse would be a mortal risk inside Sarvis’s body, the right to flush it out as well. Abortion techniques don’t target the umbilical cord, but that’s just logistics, not a consequential point. The key right on the mother’s side is the right to withhold biological inputs from a claimant, any claimant.
A right to sustenance (over and above life) is not universally accepted in American society. If it were, there would be Good Samaritan laws and right to medical care guaranteed in the Bill of rights. It is inconsistent to selectively insist on it in the case of a fetus, but not an adult. The argument from viability (as opposed to sentience, which evolves continuously) also implies abortion rights should be extended till birth. It is a discontinuous change in relevant factors.
The kind of intimate biological dependence involved in the abortion debate is rare in other interpersonal problems like ordinary murder, but there are a few parallels. Think conjoined twins (two brains, shared heart). Is it ok to give normal life to one child rather than an incredibly burdensome life to two (sort of like a more onerous lifetime pregnancy)? What about friendly vampires who can only survive on live blood?
The one pertinent objection is the issue of causation. In my hypothetical, Sarvis’s predicament is not his doing, while pregnancy is caused by the mother’s choices (although that’s not as easy as it sounds, since if she kept her legs together, the fetus wouldn’t be there in the first place, so it’s fundamentally different from endangerment of a pre-existing viable human). Nevertheless, responsibility lies on a continuum (from rape to ruptured condom to miscalculation of the cycle to wanton horniness), and since the state is in no good position to assess the particulars of individual cases, the argument for banning abortion on the basis of causation is rather weak, even if it were granted in principle.
As to equal rights for fathers, yes, post partum. It makes no sense to talk about equality at a stage when nature’s arrangement is highly asymmetric. Come back with that question when humans develop the capability of sea horses.
You can check the list of Sarvis’s premises. This line of reasoning respects all of them.
P.S. Demo’s argument that nature has its own abortions (failure to implant, miscarriages, etc.) is silly. People die of accidents and diseases all the time, but that’s no reason to allow them to be killed deliberately by other humans.