I'm asian and I don't have a particularly big penis and I find it hard to relate to 99% of porn out there... There!! I said it, laugh all you want.
Anyway, I also consider it the same as the old "alcohol-get-her-drunk" trope, too easy of a plot device.
You guys know any mom-son stories that has this.. feature..?
It's 4 inches btw, 5 inches on a good day, it's enough.. our asian scientists have confirmed this to be true