They might be upset about this now, but they'll eventually get over it. It's just a big shock to parents when their young daughter says she's pregnant. I'm sure they're not going to throw you out if you have nowhere to go with the child, even if they were to threaten to do so. They'll come around eventually - as poohbear said, they can't force you to have an abortion. Unless they sedate you and assault you with a coat hanger, there is no such thing as a forced abortion. You do what you feel is right in your heart, and do not do anything to please people...Not even your parents. Tell them to go to hell - it's your life, not theirs.
I'm sorry you're going through such a hard time - it's no paradise having 3 people breathing down your neck to have an abortion. Your parents are being major jerks right now, but honestly, they aren't doing it to make you upset - they want you to live your teenage life normally before taking such a tremendous step in life. I hate to stick up for them, but it's true. Either that, or they don't want to have an infant around since they aren't exactly sleep aids.