Here's my question: Since the fetus has a potential to become an independent living thing, does that mean that every time I wack-off, I'm killing whole generations of young?
I'm going to make up some numbers because I really don't feel like researching the issue, but I'd say it's a matter of probability. Nature dictates that people waste a lot of reproductive resources, and that's just how things are, so no, you're not really killing whole generations of young.
Only 1 sperm can fertilize an egg, so the probability per sperm that it will fertilize an egg and create a human is probably around, oh 0.00000000000000000000000000000000001% (answer truncated because I don't want to mess up the message-board width--but really, think about how small the probability is per individual sperm). We exist in such a way that it's a fact of life that just about all of the sperm men produce will be wasted.
The probability that an egg will be fertilized and become a person over the lifetime of a woman is also very, very small (0.0000000001%, let's say). Again, it's a fact of life that not every egg will be fertilized.
A simplistic way of obtaining very rough percentages would be to say that on average, every man/woman pair has n children. So for men, sperm success rate would be [n / (total sperm produced] and for women it'd be [n / (total eggs produced)]. As mentioned above, these numbers are very, very small.
However, the probability that a fertilized
egg becomes a person is probably close to 100% for a healthy woman in the United States. That's probably where the argument you're talking about comes from.