I have not read all the replies to this post but.
A few years ago I read a study (no idea where to find it now) that said men who believe they are well endowed tend to tuck it right (not all but in the high 70% I think) . There were a few reasons respondents gave, the two I remember were there is a perception that a bulge to the right is more noticeable and that most people are right hand dominant and when you pull a woman in close to you with your right hand your right leg is more likely to be between her legs so her inner (more sensative neurological) thigh would be pressed against the man's bulge letting her know he was better endowed.
Where men who don't believe they are well endowed are less likely to overtly expose their size.
Grains of salt