Because pro-lifers tend to call pro-choicers, "pro-abortion", "pro-murder" and other such things, as if we're all promoting abortion and advocating the killing of babies.
Pro-choice describes the political and ethical view that a woman should have complete control over her fertility and the choice to continue or terminate a pregnancy. This entails the guarantee of reproductive rights, which includes access to sexual education; access to safe and legal abortion, contraception, and fertility treatments; and legal protection from forced abortion.
I am all for proper sex-education, affordable and accessible birth control - so that less abortions will be necessary. I don't promote or condone abortion as a means of birth control. However, what if my birth control were to fail? I don't believe that you, especially as a man, have any right what to tell me to do with my body.
When you've been a woman, and better yet, when you've been pregnant, then you would have a better understanding of what women go through both emotionally and physically, and you could make more educated statements on your viewpoint.