Why do you need to have a step-through frame (which is a more appropriate term than "girls frame" because they can also be used by boys). That design seriously compromises the frame's strength, which is why very few quality bikes are made using this design.

If you want a step-through frame because you have restricted flexibility, then it is totally understandable and I wouldn't question it, but if it is due to style/fashion, or due to not understanding all the factors that should go into this decision, then I would urge you to reconsider the requirement for a step-through frame.