The best first time reaction I've ever gotten is also the only one:

My 2nd (now ex) girlfriend (although this would be right before we actually got together) and I were among a group of a dozen or so people sitting around, getting wasted at somebody's apartment. There was less seats than people, and at some point she sat on the corner of the couch I was sitting on... and somehow ended up sitting on my leg eventually. A little while more time passes, we get a little more wasted, and her hand bumped my already engorged bulge (I hadn't exactly had a lot of girls on my lap... hell, I still haven't lol)... she looks down, stops, sorta grabbed its girth lightly, and looked at me shaking her head slowly with mouth a bit open in disbelief. I had just enough 'narcotics' in me to just cockily shake my head up and down. She later claimed to have known before that though - it would make sense, but this is a better story I'd say.