The topic of respecting a child's physical boundaries has occasionally been a topic for discussion in my parenting cirlce of people. It does make sense to allow the child to choose whether (honestly, English language, this word makes life hard for non-native speakers!) they want to be physical with people or not. In my house, greeting guests and seeing them off is a must. Hugging, shaking hands or sticking to a spoken greeting or goodbye is a personal decision. Once, I remember a grandparent about to leave say: "Can I have a hug?" to one of the kids, around two or three at the time. When the boy in question declined, the grandparent said: "Then I'll just take one," and hugged anyway. The wee one threw quite the tantrum, and the grandparent stood by, puzzled, and said something like: "I've never thought about this before, but he has a good point. If I ask for a hug and is dismissed, it's not right to hug him like that." I think we should teach our children to expect that other people respect their physical boundaries, and to respect other's in turn. It just makes life easier for everyone.
As for myself, I like to hug and be hugged as long as I have a trusting relation to the other hugger. The adult-child relation is a very grey area, though. I remember going from deeply fond to creeped out by my instrument teacher back in my pre-teen years. We had a very good relation to begin with, I think he saw me as a bit of a substitute grandchild. He'd occasionally give me extra lessons on a Saturday and lend me books. With time, though, he took to bringing chocolate bars to my practice lesson and offering it in a playful "I'll trade it for a hug". Today, nearly thirty years later, the thought of his nicotine-stained fingers and the hugs makes my skin kreep a little. I wish, back then, that I had had the integrity to realise that no, I did not want those hugs.