The issue here isn't really about size. For centuries, men have been led to believe a larger member makes them better in bed, more fertile, more masculine, or more attractive to a partner. None of these things are true.
Gay and straight partners of men have complained that longer penises are uncomfortable to accommodate and make some positions impossible.
Straight women say they're more interested in girth than length (although guys don't always know this). Men with larger knobs also are more likely to have problems getting and keeping erections.
You also can't rely on a big cock to get you by in the bedroom. Research suggests men with larger penises don't try as hard to please their partners - leaving both parties dissatisfied.
The reason sex therapists or medics are unwilling to discuss penis size is because men are still often not satisfied to hear their penis falls within an average size range - they want to be bigger than average.
Even if you tell guys they're okay, often their body image worries means they're still anxious. A recent US study showed that men seeking penis enlargement consistently overestimate the average penis size of other men and drastically underestimate their own size.
Men get penis hang ups since they usually only see other penises in the changing rooms or toilets - where they view their dick downwards and everyone else's front or side on making other guys seem bigger. Or they compare themselves with porn stars, where the actors are specifically chosen for their size - and filmed from flattering angles.
