There is a basic truth, that regardless of how straight or how gay a guy is, we are all fascinated by cocks. There's the competitive thing where we want to know who's bigger than who. That can come from insecurity, or just cameraderie. It's often just a male bonding thing, which has happened with a fair few of my mates. At one party, all the guys gathered in the kitchen, stood in a circle, and all got our dicks out. Everyone was soft, there was no "gayness", no erections, no jerking. It was just a funny moment that we still joke about. It doesn't mean they wanted a big circle jerk, or anything sexual. In fact, it was one of the least sexual examples of male nudity I can remember.
Ask most straight girls, and they can probably tell you all their close friend's bra sizes. Girls compare. If a girl has a problem, if she's not sure about something, or heaven forbid, thinks she may have a lump, few will be shy about asking one of their closest friends to offer advice, or even to check that everything is "alright". There's no sexual element to it, just closeness and friendship.
Most of my straight friends are the same. They're not gay, but they're comfortable enough that they're not gay to make a joke about it (for instance, one of my str8 mates told me one day "If I ever turn, I'll want to suck your cock, Irvy"). The problem is that so many (esp gay guys) view this kind of behaviour as a sign of "curiosity", when in fact, it shows a complete lack of gay tendancies or curiosity, it just means they're as comfortable with being str8 as I am about being gay.
So yeah, make a joke of it, whip your dick out, and don't expect him to suck it. Chances are, he'll make a joke, or say something like "I thought you were packing but I wasn't sure", and that'll be the end of it. However, it may well open up a chapter of your friendship were you can be closer and more comfortable with each other, and able to confide in and trust each other more than before - without getting your dick sucked.
