Regarding our experiences (both wife and husband) the size of a cock does matter, but only to a degree or extent.
Size matters when the male organ is unfortunately too small. We mean the clinically small ones, not talking about the 5 incher which is probably the median or average for many people and is very able to satisfy.
Size matters when the male organ is big and thick, because it is alluring for most women. They expect more pleasure, excitement, and to pose proud before other women as a conqueror of the big scary one-eyed monster. Yes, a big cock male is indeed some kind of a trophy for playful wives.
Size matters when the woman is satisfied with her partners sizes and mainly because they know how to handle their girth and reach in her favor, making her cum good.
So, a big cock brings orgams and probably more allure and excitement than an average joe one (not an offense, husband has a 6 inches and is fine). Therefore, it is not a myth.
The myth is to believe that just a big dong hanging between any male legs would be enough to transform him into a Bull or Casanova!
