I understand what it is when people want to touch the belly. I never did touch anyone elses belly without their permission first. But, as a pregnant person, it helps to automatically assume that everyone you talk to wants to touch the belly...because it's beautiful, and cool, and wonderful. They want to be a part of something so awesome that even a touch on the belly makes them feel the miracle. When I was pregnant, I didn't care if people touched my belly, because I understood that want, so it didn't bug me often.
The few occasions when I did feel uncomfortable about someone touching me, I could sort of tell when people were going to do it. Try this, it works great! Quickly put your arms around your belly as if I were hugging your own belly, and make your hands really big so that there isn't any room for anyone elses hands. Move them around on your belly alot so that if a person would try and touch your belly, they wouldn't find any room for their hands. If you have something in your arms, just move it inbetween your belly and the offending person. Then, a offender can't get to it. Then, the belly isn't so..."Out there" and open for free touching. Good luck. :) You could also turn sidways a little, and with a combination of your hands moving or an object infront of your belly, there won't be room for a person to touch it. You'll be surprised at how well it works to avoid it entirely and it'll greaten your need to protect your young even while in your belly. It will strenthen your perception skills. Good luck.