Answer:
People think that it is acceptable to perform abortion because sometimes, the baby is not theirs, and in many cases, young women have been touched illegally(can't use that word), only to discover that they are going to have a baby. Now of course, you wouldn't want to have a child that is the son/daughter of someone who touches people inappropriately so people do abortions because of that.
Also, some families do not want to have children, and they might have forgotten to use things that prevent people from having children(can't talk about that stuff here, can we?), and then they have a baby starting to grow. In this case, people who do not want to have kids choose to have an abortion because it is their choice, and it doesn't really matter what other people think, and if they don't want to have kids, then it is their choice.
Note: If you want more in-depth detail, just ask. :)