There is not hurt in the world like when you have a child from whom you are forcibly separated. You should know that there probably will be no respect and real love in this arrangement and neither of you will feel any real committment or obligation to each other.
Good luck, but I think you will emerge from this jaded and cynical about relationships. It's too bad your girlfriend doesn't have enough respect for you or herself to insist that you get married. Just don't have kids.