I just want to jump in and give the 'back in the day' answer. This was never odd or creepy when I was growing up, but also, never did someone just reach out and start rubbing.
I've seen it many times--a person, most often a friend or relative, will comment on how you're 'glowing,' and then they would ask permission! More often than not, they would ask the mother if the baby was kicking, but then if they wanted to touch the mother's stomach, they 1) always asked first, and 2) it was to see if they could feel the baby (men especially, as pregnancy is something they can't experience first-hand), and 3) there was never any rubbing involved. It was a touch. If given permission, they placed their hand on the mother's stomach in the hopes of feeling a kick, or the baby's hand or foot. At no point in time did the rubbin' commence.
As a society, we've just gotten entirely too weird.