I don't know if all women feel this way but I know an erect penis is probably the oddest thing in the world the first time you see it. Not odd like weird or gross but odd like "Oh my God, it is standing up on its own!" Women have nothing comparable to that. Everything about a women's sexuality is so subtle. I mean guys are always asking how to tell if a woman is excited or having an orgasm. An appendage standing up on its own pointing at you is anything but subtle and I think when you are a young girl it is really overwhelming.
Plus, it's not like he can hide his desire. He can't stand there with an erect penis and go "Nah, I'm not too attracted to you." So that is kind of odd cause it's like someone screaming at you "I WANT SEX WITH YOU RIGHT NOW!".
Also since guys have so much introduction to sex with porn at younger ages and most girls have either none or very little, it's just completely uncharted territory. So there probably is a huge sense of awe in looking at a guy naked and turned on for the first time cause you have no idea what you are looking at! You have no idea what you are supposed to do with it. Does he do something with it? It can be completely confusing.
I didn't laugh cause I'm not an awkward laugher but I do understand what MoreLikeFlying means by laughing cause the whole experience is like a shock to the senses.
But I played it cool. :) I gave him my best "Come over her big boy look." when all I wanted to say was "Can I stare at your penis for like an hour cause I'm just fascinated by that."