Here are some things to consider as you sort through the confusion. First of all, you're both still quite young. Over the years, feelings can change in a relationship and toward life in general. Today, you don't want more little ones. In five years, you may feel very differently.
Life has a way of unfolding in ways that are entirely unpredictable from today's point of view.
Many times, I've counseled couples who were sure about the issue of children before marriage, only to have contradictory feelings emerge through the course of their lives together. Yes, she must consider that by marrying you, she may give up the possibility of her own children. She must realize that as time goes on, she may long for her own babies, and that this will be a source of struggle between the two of you. Not that it has to go that way, but that there's a good possibility that it will. You must consider that by marrying her, you are choosing a difficult path also.