I should tell you from the start that none of the following comes from my own head. I was listening to a John MacArthur sermon on the radio one day and he made the below point that I think is darn near incontrovertible.
Here is the basic thesis: the abortion debate is about sex, not abortion. The reason that there is any question about when a fetus becomes a real human being (and thus, whether it is ok to kill it or not) is because sex is so fun. The baby is seen as an unfortunate side effect of the romping good time of copulation.
Think about it like this: if human procreation happened through, say, an adult male and adult female running a mile together while holding hands, there would be no question about whether the resulting fetus was a human 3 months into pregnancy or not. Or take it a step further and imagine that the female had to swiftly kick the male in the groin for conception to happen. If the process was no fun, the result would be no question. The abortion debate is thus not the abortion debate at all; it is the sex debate. (NB: these counter-examples are not from MacArthur!).
Obviously I've made no scientific argument here and my only appeal is to human conscience. But it is intuitive, right? Sinful humanity wants to have sex, and they do not want any consequences or limits. All of this serves as one more reminder of the utter pervasiveness of sin in our world. It is a stark realization that if we must kill babies so that we can have more sex, so be it!