Thanks for the question, Mark.
Like MichaelSouth alluded to, this is tricky because it’s not simply a matter of refraining from murder (let’s stipulate that that’s what is is), but if you have an unwilling mother, you run into some problems. E.g. what if there’s a pregnant woman who is a chain smoker and drinks whiskey every night? Should she be forcibly restrained for 9 months? It’s a lot trickier than a rule saying, “If you murder someone else you go to jail.”
I guess I have two main ways of responding to your concern:
(A) I don’t think it’s useful to contrast an “ideal minarchism” with an “ideal an-capism,” since I don’t think an “ideal minarchism” is stable. So if you’re saying you want a State that has the power to tax, monopolize judicial rulings, etc., so long as it limits itself to things you consider to be defense of life and property, I think that’s being very naive.
(B) The ultimate moral principle you seem to be espousing is this: “I, Mark, plus lots of other religious people, think abortion is murder. But plenty of other people don’t. If we voluntarily fund law enforcement efforts, I am worried there will not be enough money devoted to punishing abortions. Therefore, I think it is moral for us to take money from the other people against their will and use it to stop abortions, even though they wouldn’t voluntarily spend what I agree is their money that way.”
I realize those aren’t the words you used, but isn’t that basically what you are saying? If so, no, I don’t think that is morally defensible, even though I agree with you that abortion is the killing of a human.