Ben Affleck's Junk -- No Big Deal ... to Matt Damon, Anyway

Most of us had to pay to get a tiny glimpse of Ben Affleck's weiner in "Gone Girl" ... but his best buddy Matt Damon got to see it for free ... and many times, according to him.

Why exactly he's seen it that often, we're not sure, but one thing's for sure -- Matt seems unimpressed.