No, feeding a child is not part of reproduction. Once the child is born, congratulations, you have reproduced. Rearing the child will involve feeding it, which is best accomplished through breastfeeding but also suitably done through bottle feeding, or any of the other artificial methods historically used. Is a baby bottle now a sexual item that should only be sold at adult novelty stores? At what age do we consider the child old enough that whatever they use to eat is no longer a sexual object 2, 5, 8, 12, how about 18? Hell as a mature adult, rearing further offspring, maintaining my life is obviously part of reproducing right? So can I count all my eating utensils, dinner ware, and cooking implements as sexual objects?
Honestly trying to say that breasts are the only purely sexual organ because they are used to feed children is an enormous stretch. Besides what about the Ovaries, Uterus, and vagina? Exactly what besides sex are those organs for? I suppose the argument could be made that they produce hormones, but the major point of those hormones seems to be sex drive and femininity.
I'm not sure that a narrow waist is actually an indication of a good birthing physiology. I believe it is actually wide hips which people think indicates a lower risk of mortality in child birth. However by constricting the waist artificially it makes whatever hips are present seem wider.