i think this is in the same category as 'bump touchers' im quite forward when someone annoys me so i touch their belly too and ask if they are going to have any kids, as though its perfectly normal. they dont normally do it again.
i think people just like to acknowledge your pregnancy when they speak to you and are limited in what exactly they should say. I have found that people always comment on the size of my bump although the comments they pass are clearly just for something to say - 50% say its massive and 50% say im small for 21 weeks!
People are just so socially awkward they say ridiculous things!