Or, ya know, it's just a guy thing. Like so many guys do this. It's a subconscious way of drawing attention to their crotch. It's a manliness thing. At least that is what I learned from my book on reading body language.
Captain America: Evolution of a Super Soldier Infographic - see all of Captain America costumes from 1941 to today! I love how they put nomad in there, nomad is what cap becomes after he gives his shield to stark in the civil war comics
Movie Review: "Captain America: The First Avenger" (2011)