Why are people pro-choice?

Some people say oh its not a baby, um you can only get an abortion at 3 months, that thing is already formed and living, it is murder, I only believe in abortioin if you were d or you are such a pathetic excuse for a human being that you have no busines having kids, but then i think it should be mandatory u get a vasectomy or tubes tied, but you should pay for your consequences, you arent special enough to be forgiven so easily. How is this ok?