But our things are merely out in front, totally out of the way of other things - and they certainly do NOT interrupt the actual means of perambulating, the legs. Yours are inbetween as well as in front, and get rubbed - or more importantly, the thighs get rubbed. Kind of awkward if you ask me. ;-)
Boobs get in the way of things. They touch people you don't intend to have bodily contact with. They get in the way of putting on seat belts. They keep women from being able to run freely because they bounce and throw off your balance.
They may not affect the means of walking, but they affect other things, your interaction with your environment.