As to the last couple posts here, I think it does make a BIG difference whether the guy whose 'overtures' are in question does or does not know (or suspect) that the other guy is gay.
If he does NOT KNOW, then I would err on presuming that the guy simply has more flexible affection habits than most, and that his affection is probably just platonic (poorly attuned to society's norms maybe, but platonic).
However, if he DOES know that the other guy is gay, well, then I think the likelihood is very hight that when he makes societally 'questionable' affectionate gestures toward the other guy he is either:
1) Flirting or making a pass (overtly or subconsciously); or
2) He is being cruel and/or manipulative (again, either overtly or subconsciously).
So, if you are the gay guy on the receiving end of a perplexing gesture of affection like this (from another guy who does know you are gay), it seems to me the best response should be the same in either case -- that is, reciprocate with a clearly sexual gesture of your own towards him. If the scenario is #1, you might be on the road to something hot together. Alternatively, if the scenario is #2, you'll just freak the bejeezus out of the little fucker, which is what he deserves.
Just my 2 cents here.