I think personally that you should stick by her in her decision... I'm not pro-life as such I agree with abortion in certian circumstances but maybe this isn't one of them if the mother wants to keep the baby.
Talk to her and tell her exactly how you feel and make her see your side of the ordeal... You should really have used other forms of contraception if it was only a one night stand as she could have an std or something.
In a way (i dont know the laws where you are) but here in the uk you have to stick by the mother as even if you are split up the father has to pay for some of the childs upbringing... It does take two, you are right, but whatever your mother says and however much you do not want a kid you were the other member in the duo. Good luck, let us know what happens.