As demonstrated in my holding hands story, the rules of contact between two people are a little different here. Girls routinely hold hands (they even interlock fingers) with other girls and it doesn't mean they're gay with each other. Every night you'll see drunk, middle-aged men arm in arm walking home after their nightly drinking sesh.
Even slapping the butt is acceptable in non-sports-celebratory situations. Yesterday, I witnessed some guy sitting on the floor of the metro. As he got up, he used his friends pant crotch area as an anchor point to lift himself (What, no handrails?).
There's a game the boys like to play. It's called 똥침 (Ttong-Tcheem). From wikipedia: "It is [an act] performed by clasping the hands together so the index fingers are pointing out and attempting to insert them sharply into someone's anal region when the victim is not looking." It goes without saying that the victim is clothed, so no actual -uh- insertion takes place. Nevertheless, it's really gross. But the kids seem to derive a lot of pleasure from sticking their fingers in each other's butts. Most of them grow out of it by their mid-twenties though.
Okay, so we've established that it's okay to pretty much touch someone (of the same gender) anywhere, even the inside of their butts. But what about opposite sex physical contact? Well, it's simple, if you're touching someone who pees in a different position than you, you're dating that person.
Last thing, you don't EVER want to touch a dude's head, especially if he's older. If you do, get ready to get your head slapped and yelled at forever. It is VERY disrespectful to touch an older person on the head. I don't know exactly why. The only reason I found was because the head it the highest, therefore most sacred, part of the body. But I'm not 100% on that.
Here's another (somewhat) crazy Korean superstition. Do not let your hair get wet when it rains. You will eventually go bald if you you do. You see, the rain here is apparently very acidic, or at least very polluted. Anyways, Koreans believe that prolonged exposure to water falling from the sky leads to hair loss. So when it rains, every single Korean person (guys and girls) scurries to hide under an umbrella. If an umbrella isn't available, anything else will do (newspaper, books, dogs, your boyfriend).