Some thought experiments on "potential life"

An ex-theist emailed to say that, although he has made a lot of changes to his thinking regarding gay rights and race issues since abandoning his theism, the abortion issue still bothers him.

The human egg and sperm are not in and of themselves able to "live" and reproduce/multiply on their own. Once they are joined, something happens that causes them to "become alive" and the cells will them multiply on their own without any external influence other than feeding off the body of the mother.

The glob of cells will in the vast majority of cases eventually become a human and the progression of its growth can not be physically stopped by the mother or father without the prescribed use of a poison pill, or physical instrument where a doctor must physically cut it or smash it until the growth stops.

I'm no legal scholar, but I can not see how this action can not be defined as anything other than "killing" an immature human.

Rather than just send him off to another site, I gave a little more thought to the implications of requiring the care of a fetus on the basis of it being a potential future life as soon as the sperm and egg join. For starters, you can't go wrong reading Carl Sagan's essay on abortion from Billions and Billions:

Despite many claims to the contrary, life does not begin at conception: It is an unbroken chain that stretches back nearly to the origin of the Earth, 4.6 billion years ago. Nor does human life begin at conception: It is an unbroken chain dating back to the origin of our species, hundreds of thousands of years ago. Every human sperm and egg is, beyond the shadow of a doubt, alive. They are not human beings, of course. However, it could be argued that neither is a fertilized egg.

In some animals, an egg develops into a healthy adult without benefit of a sperm cell. But not, so far as we know, among humans. A sperm and an unfertilized egg jointly comprise the full genetic blueprint for a human being. Under certain circumstances, after fertilization, they can develop into a baby. But most fertilized eggs are spontaneously miscarried. Development into a baby is by no means guaranteed. Neither a sperm and egg separately, nor a fertilized egg, is more than a potential baby or a potential adult. So if a sperm and egg are as human as the fertilized egg produced by their union, and if it is murder to destroy a fertilized egg--despite the fact that it's only potentially a baby--why isn't it murder to destroy a sperm or an egg?

For context, here's support for Sagan's claim of the frequency of spontaneous abortion from the University of Ottowa:
"The incidence of spontaneous abortion is estimated to be 50% of all pregnancies, based on the assumption that many pregnancies abort spontaneously with no clinical recognition."

So if a fertilized egg is more likely than not to not grow into an adult human being, why draw arbitrary lines in the sand saying that it becomes murder in that particular moment?

For the sake of argument, I'd like you to imagine that time travel is possible in order to consider the following eight thought experiments.

1. You go back in time and deliberately prevent somebody's parents from meeting. To be concrete, we'll call him "Biff". History has now changed and Biff is never born. Have you killed Biff? (If you're like me, the answer is "Maybe. I'll have to think about it a bit.")
2. Suppose that, instead of preventing Biff's parents from meeting, you go back to the night of his conception and strike up a conversation with them. The three of you have a delightful time until late at night, and they never get around to having sex. Again, Biff is never conceived. Again, have you murdered him?
3. Now suppose that Biff's parents were already actively planning to have a kid, and so they go at at the next night. A child is conceived but -- due to the statistical issues involved -- a different sperm implant in the egg, and the genes expresses themselves in very different ways. Returning to the present, you find that Biff doesn't exist at all. In his place, his not at all similar brother Griff was born. Is Biff now dead?
4. In order to fix the timestream, you travel back and prevent yourself from meeting Griff's parents, thus restoring the original history. Biff is born and Griff is not. Have you now killed Griff?
5. You and your partner discuss having a child of your own, and almost decide to do it, but in the end you decide that the cons just barely outweigh the pros. Had the argument gone a little bit differently, you might have had a kid. Have you killed your future child?
6. You (or your wife) are pregnant, but there are complications -- possibly not fatal, but definitely not something you would like to deal with. You agree to abort the baby and try again. The original fetus is never born, and the new baby is healthy, happy, and grows to adulthood. If you had chosen to bear the original fetus, you wouldn't have wanted any more children. By deciding not to have the abortion, would you have been killing the healthy baby?
7. Some religious groups teach that child bearing is a responsibility and a duty. Protection of any kind is never allowed during sex, and therefore they have fifteen kids. Compare them to a couple who bear two children by choice and then use protection for the rest of their lives. Have they killed the other thirteen children that they might have had? Do thirteen murders simultaneously occur as soon as the man gets a vasectomy? What if they decided to have no kids, is the murder count now bumped up to fifteen?
8. Similarly, is an abstinent couple committing murder by giving their future children no opportunity to come to life?