donesu, you need to involve your parents NOW. You need an authority figure on your side to help you deal with this girl and her family. I don't think this was an accident, it sounds like she got pregnant on purpose. I just can't see any girl turning a guy down who offers to wear a condom. She should know that the pill doesn't protect against std's for example, so there is always a reason to use a condom.
You need a parent on your side to protect you from this girl and her family. Your father should go over to their house and make it clear to them that you were not irresponsible, that you did everything you could to prevent this from happening, that she, the ex, refused the condoms, and that you have no intention of ruining your entire life because she chooses to have a baby. CHOOSES. It is a CHOICE.