According to The National Library of Medicine website:
Most men believe that the average length of an erect penis is greater than 6 inches (15.24 cm). This belief is due, in part, to several often-cited studies that relied on self-reported measurements, with means of about 6.2 inches (15.75 cm) for heterosexual men and even greater for gay men. These studies suffered from both volunteer bias and social desirability bias. In this review, the combined mean for 10 studies in which researchers took measurements of erect penises was 5.36 inches (13.61 cm; n = 1,629). For 21 studies in which researchers measured stretched penises, the mean was approximately 5.11 inches (12.98 cm; n = 13,719). Based on these studies, the average length of an erect penis is between 5.1 and 5.5 inches (12.95-13.97 cm), but after taking volunteer bias into account, it is probably toward the lower end of this range. Studies show that a majority of men wish they were larger, with some choosing penile lengthening surgery. These surgeries are considered by the American Urological Association to be risky. Most men seeking surgery have normal sized penises. Counseling with factual information about penis size might be effective in alleviating concerns for the majority of men who worry about having a small penis.
In other words, many men have difficulty admitting their true size. Most men believe the average to be bigger than the reality. Most men also wish they had a bigger penis (68% on WebMD). Men have obvious insecurities about their size and are willing to lie rather than to reveal their actual size even to an anonymous survey.
Most women according to this thread are looking for something that most men do not have. It also suggests to me that those men with big cocks must be pretty busy if everyone wants what they have and there are only so many big cocks to go around. So, women that only want 7 inches or above will spend more time looking because numbers wise, those cocks just aren't out there. But, cuckold/hotwife/stag and vixen lifestyles are also in the minority so statistically this discussion could end up as analyzing outliers.