I think MrMoe makes great sense.
Pregnancy is the worst reason in the world to get married if you don't love each other. If the only reason you are going to get married is because of the baby, you will end up resenting each other, rowing all the time, and that won't be fair on the baby. However, if you love each other and want to be together, that is a different thing.
Please don't feel you have to do anything that you feel uneasy with. If your gut feeling is that marriage is wrong - don't do it. You are going to have to grow up pretty quick and face the condemnation of all around you, but you can get through this. Whatever happens, if your girlfriend keeps the baby, make sure you support it financially and (if she lets you) emotionally too. Take time to think about what you want to do.
Hope it works out.
xxR