Why do people think its ok to touch your belly when you’re pregnant?

Actually the title was all I wanted to say about this topic, heh. I am actually a rather open person and probably more touchy feely than average by Asian standards. I’ll put my arm over the shoulders of people I haven’t known for very long but feel are nice people and am ok with a greeting/parting hug. But what’s with people who are not really friends coming over to rub your belly, and guys at that. Ladies I generally have not much of a problem with, although I can imagine finding it strange if some professional contacts do it. But men! Why do they also think its ok??