Answer:
You don't need to worry about it, its not very hard. If you're in a situation where you want to rub a guy's cock, just put your hands on his pants...you should feel it if its hard, and run your hand over it. If you want to do it with his pants off you can rap your hand around it and stroke it...don't worry, if he doesn't like what you're doing (which he probably will) he'll help you out.