Here is a better example of how bad it CAN look. Sitting down and leaning forward makes it look the worse.
Slight cases can be just as detrimental as serious ones. Slight cases result in paranoia of how much people really notice at any given time. Because some shirts do an excellent job (as in makes it look like I have a flat chest) I've become HIGHLY selective in what shirts I wear. Also because making the nipples hard makes it look like I have no issue whatsoever (moreso than more severe cases) I'm constantly having to use that technique when I take my shirt off.
I unfortunately went to a middle school/high school where literally no other male had this problem. Sure there were a few fat kids but they had pretty uniform fat all over. Most males, however, were very athletic and had perfectly flat chests. As a result, this "slight" case was very apparent.