I totally feel you on this one! (Get it, feel you?) I too dislike the belly rub of a pregnant woman and wasn't shy about my opinion either time I was pregnant. I know that it comes from a good place, but I feel it should be asked if it is ok, then the mom-to-be can decide. One of my mom's female friends kept doing it to me, even after I had expressed my opinion, so I patted her tummy right back and it ended it.
Another time I had a man not just pat, but put his arm behind my back and give a circle rub on my belly. He asked "how's it going in there?" So I patten his butt and said, "better than in here."
I felt very vulnerable when I was pregnant and kept seeing news stories about attacks on pregnant women, which just worsened my feelings of vulnerability.
If you see a hand reaching out, it is easy to take it in yours and give a shake or to simply step back. These usually send a polite but firm message. If you need to be more direct, please feel free. You need to be as comfortable as possible while pregnant.
Just my thoughts! Congratulations!