I think a lot of them are fishing for the "Oh my God" reaction - sometimes its warranted, most times not, since most men are average (definition of average!).
I think men think we are turned on by the mere sight. Sometimes, but not usually.
What turns me on is the guy who is casual about it - they guy who isn't afraid to chenge clothes in front of me, but doesn't make a point of shoving his cock at me; the guy who will go skinny dipping with me and friends who isn't also parading himself around; the guy who might even be a little embarrassed when he gets a hard-on and it is visible in his pants; the guy who, when making out for the first time with me, doesn't automatically move my hand down there - I like to discover things for myself, at a pace that matches the moment and the temperament of the guy.
I've been called old-fashioned - I like it when the cock-sighting is part of a natural discovery and uncovering process, rather than a "hang it out and shake it at me" thing. I might gasp in wonderment if a truly magnificent one is exposed - but I'd rather gasp in wonderment as I unzip my new friend, or get a glimpse of him when we skinny dip or something .....
Bottom line - flashing your cock at me might get an "oooh, that's nice", but rarely will it turn me on.