Well, adoption is one thing. Basically, anyone who adopts really, really wants a kid. So they're not going to be bastards. But if you have a kid as a result of faulty contraceptives or something, and do not abort though you want to, that child is going to grow up as "an accident". Oh sure, the parents will love him and all, but not as much as if he was an intentional child.
Well, thats the theory anyway.
I think of it like this. A fetus is not "alive" becuase all living this have an interaction process with the world around the, They interact. Animals, tree, people, they all do it. A fetus, however, has no give/take, it just has take. It sits there as takes from the mother, and has no sort of "action" occuring. As opposed to when its born, and it starts breathing, eating, learning and all that.
Not the strongest arguement, I admit, but its someting to think about.