Even though most women use contraception, accidents still happen. Why should the woman, who tried to be responsible, be told she should be responsible and have a baby she isn't ready for?
A 12 week old fetus doesn't have the brain capacity to feel pain. Look up 'cerebral cortex'.
Why? Because that's life. When you take risks with existing consequences, you don't get to act like you didn't have a choice. When you have sex there is the possibility you will get pregnant. We've made those possibilities pretty slim, but you still have to face the consequences. Here's a comparison. If the man and women have sex and she becomes pregnant and keeps the baby, why does the father have to pay child support if he favored an abortion? Why should a man, who tried to be responsible, be told he should be financially responsible for a baby he isn't ready for?
Pretty sexist when you use that logic.
Also, about your claim that a 12 wk old fetus doesn't feel pain. The only reports we have show that the thalamus and limbic systems account for most pain reception and are developed at week 17. The cerebral cortex accounts for higher order thinking and to act like that's responsible for pain reception is as moronic as saying animals don't feel pain (since most non-mammals don't have a cerebral cortex). We have proven that they feel pain at week 17, but we don't know if they feel it earlier. At week 12 a fetus responds to external stimuli, which is pretty strong evidence that they feel pain.