To answer your question directly I'd say it happens a LOT! lol! Certainly more than girls realize because how would a girl know for sure unless they noticed a tent growing in his pants or a lot of unnecessary adjusting himself down there lol . Not that it happens real often, just that it happens more than they think. It depends on the situation the guy is in as far as other girls being around and how hot they're dressed and if they're being teasy/flirty and if that appeals to any guys that happen to be around. Guys are sexually stimulated visually much more than girls, maybe equal to how much girls are stimulated emotionally more than boys. Also, some guys are no doubt more open to that than others as they watch certain girls more closely. lol!.