There is an very common occurring theme in Asimov's work: an obsessive need not to touch people considered unclean, sometimes justified sometimes not.
1) The Caves of Steel (Robots series):
The Spacers (former Earth colonists living in Utopian conditions throughout Space) have long eliminated all disease in their worlds, but their immune systems decayed as a result (the common cold can kill them), so they are downright paranoid when dealing with normal Earth people (who live in entirely different conditions)—forcing them to take thorough showers and burning all materials they come in contact with.
Source: TV Tropes: Terrified of Germs
2) in The Naked Sun Solarians go even further by forbidding meeting in perso and touching between any two people Spacers or not:
By the time Elijah Baley visited Solaria around 5022 AD, its inhabitants had evolved an isolationist culture in which its citizens never had to meet, save for sexual contact for reproductive purposes. All other contact was accomplished by sophisticated telepresence "viewing" systems, with most Solarians exhibiting a strong phobia towards actual contact, or even being in the same room as another human. All work was done by robots.
Source (Wikipedia: http://en.wikipedia.org/wiki/Solaria)
3) In Pebble in the Sky future descendants of the Settlers are again looking down upon Earthers, this time because Earth is irradiated and they are afraid (based sometimes on the facts, other times on superstition). I can't find the exact quote where imperials avoided touching them, but they certainly were disgusted by the people of Earth whether they wanted to admit it or not.
4) A somewhat smaller example in Prelude to Foundation. The people of Mycogen, a sector in the planet-wide city of Trantor, have a lot of unusual taboos and customs, one of which involves touching:
He manages this by convincing Raindrop Forty-three to show him the prized Mycogenian microfarms, a prized source of food for the aristocracy and Mycogenians alike, then asking her to show her their religious historical book. Raindrop Forty-three accepts on the condition that Hari allows her to touch his hair; (hair being expressively forbidden in Mycogenian society).
Source: Wikipedia: Prelude to Foundation
While all of these examples may be justified with a real lack of immunity in (1) and (2) as well as social circumstances in (2), radiation and mutated diseases in (3), then religious fervor in (4), I really noticed the pattern only when I saw the following example:
5) In The End of Eternity all classes of Eternals despise Technicians (the group to which the main character belongs). They all have to cooperate with them, but not to be nice about it as well. If a Technician is passing through a hallway everyone would look away, etc. The important thing in the context of this question is that in one scene an Eternal was passing by the Technician being very careful not to even touch him. In this example, we are talking merely about professional differences, maybe rivalry and disapproval of what he does. There is no sane justification I can think of.
As presented in the examples, this really seems like quite a recurring theme in Asimov's work. Even more so than what Space Elevator is to Arthur Clarke. Now that I think about it all Asimov's stories settings have always seemed somewhat simplified and sterile.
So, what could have been the reason? Was he possibly afraid of germs or perhaps of touch? It is already confirmed that his agoraphobia was an inspiration for future Earth and Trantor. Did he think that the industrial progress would inevitably lead to that kind of lifestyle? Perhaps he was just trying to depict modern world in future, most notably racism?
Any resources, interviews, mentions, well backed opinions... anything?