lol I don't think it's because they see a girl, at least not the majority of the time. It's just like when a girl needs to adjust her boobs, guys have the same thing. For some guys it's worse then others. I have known a few guys who constantly have to adjust themself. I'm not sure if it's the underwear, or what, but obviously they are uncomfortable and have to readjust.
its just somethin guys always do I live around so many guys who adjust themselves and evn my boyfriend doe it in front of me and I personally agree that the underwear have to do with it even boxers make it move around so they have to keep adjustin