This may sound a little strange, but I have often seen gay men routinely grab the backside of another man they knew to be gay and sort of - er, massage it, just as a routine form of greeting. Sort of like dogs sniffing, I suppose, and about as suitable for public events. I lived in SF for many years, so I know whereof I speak.
Thanks. I think I'll be staying out of San Francisco. If I wanted to be "man" handled that way, I would have tried out for the Chicago Cubs.