You guys realize rubbing a pregnant woman's belly and rubbing a strangers boobs are totally different, right???
I don't get why women get so mad over belly touching.
1) They aren't any different at all.
2) So, I wonder if you don't understand why guys shouldn't grab their waitress's ass either. Yet, harmless intent doesn't make that action defensible either.
Uninvited intimate physical contact from one stranger to another is not socially acceptable, generally rude, and is kind of creepy too. Just because a woman is pregnant does not mean she had ceded domain over her body to the general public. People just lose their minds and common sense around pregnancy ladies, but that doesn't make it right. Just as in other, more immediately harmful circumstances, no means no.
If you are content letting strangers handle your belly, then you can certainly give permission tacitly by not taking action to prevent it. But, that doesn't mean that women who don't want strangers handling their body are wrong for objecting, or for getting mad when their objections are brushed aside because the person intent on touching isn't meaning any harm.