Ugh. Puns.
The best thing to do, especially if your girlfriend is in a state of mind where she won't kill an animal, but is willing to kill her baby, is talk to her about it and figure out what the best course of action is for both of you. You may not like it, but abortion doesn't have to be the answer. The child will be better off (and not dead) if you give him up for adoption. I disagree with cookiecutter's point of view on this matter. Why kill it? Why? There's really no reason unless the child is concieved through incest, rape, or there's a danger to the mother's life. The best alternative is to see the pregnancy through, then give the child up for adoption.
This is your mistake, bro. I hate to put it like that, but at the same time...it really is. The only foolproof way to not have a baby is to not have sex. Your girlfriend is pregnant, and now you have a big choice to make. I don't think that you should get an abortion. This is a huge moral issue, and when abortion is used as a means of birth control, it teaches the youth of America irresponsibility and desensitizes them to violence and, as I believe, murder. Don't kill it, let someone else raise it, someone who, perhaps, might not be able to have their own child. If the sex that conceived the child was meant to be a loving bond to bring the two of you closer together, killing the creation from that union would seem rather foolish, or so one would think.