Do you believe that if guns were made illegal in the US, everyone would give up their guns and nobody would obtain them illegally?
Nope. And we don't think if abortion was banned nobody would get them illegally either. Whether people will follow a law is really irrelevant to whether it should exist. Granted, the "People will still buy guns" argument is valid when a progressive makes a pragmatic argument against gun ownership (Which is usually the flavor that these sorts of arguments take). Libertarians, however, do not generally view the world in pragmatic terms. The question is whether an action is an act of aggression against other people. Granted, abortion is tricky and there are libertarians on both sides. But this sort of pragmatism shouldn't really enter into it.
Yes, people would still have abortions if abortion was illegal. Yes, people currently do commit murder even if murder is illegal. I wish it was illegal for the government to commit murder, but even if it were, people would still commit murder.
Guns are a completely different issue, there's no violation of anyone's rights through owning a gun, so owning guns should clearly be legal.
The difference there is that you'd kill a person in your cancer instance, even if someone believes a fetus is a human being that should be protected, they are not (and have never been) legally persons. So it's not similar.
Libertarians are not legal positivists.
It wasn't meant to be an argument for or against (although it's a reason to be against a ban on a federal level), just a snarky comment for the most part. Since I don't believe abortion should be illegal or that it's murder, I view back room abortions as making a potentially bad situation worse. I would rather women have the option to safely abort than resort to unsafe practices out of desperation. Personal responsibility and avoiding having to even think about abortion is of course best, but if a woman truly wanted to have an abortion and it was before or around the third trimester, I don't see what the issue is because I don't yet consider it to be life, just potential. Past that, I haven't made up my mind. I don't view abortion in absolutes, though. I find Rothbard's case on evictionism to be fascinating.
Did Rothbard ever go that far? I thought Block invented evictionism, while Rothbard was just straight up pro-choice. I'm not positive though. In any case, Block's evictionism argument is the pro-choice argument I have the most respect for, but I still reject it. I don't believe your property rights give you the right to kill an innocent human being who never chose to be in that location anyway. If it was rape, the rapist is a tresspasser and should have to pay whatever costs the court deem fair for nine months of the invasion of the mother's womb (I assume that would be a rather high price, at least IMO), while in consensual sex cases, the mother invited the fetus into her womb, so no tresspassing occurs at all. I only believe abortion can be in any way justified if the mother is likely to die from doing anything different.