Wow, that is something that one discusses before marriage. "Babies will tie her?" As in tie her down? To me, she is saying that she does not plan on being with him for very long. She wants to be able to just get up and leave, and leaving is easier if they do not have any children.
Is it a sin? Well, I'm not God, so it is not for me to decide that. Also, there is not enough information for me to decide who is right or wrong, for example: he could be abusive, and so she does not want to bring children in that kind of environment, or maybe they did in fact discuss it before marriage, and he agreed to the terms with the intentions that he will try to change her mind as they go. In those two instances, he is wrong, but, as I said before, there is not enough information to make that kind of decision.
Well, good luck buddy!