My cock is just under 4" erect and while my wife hasn't exactly made fun of me, she does sleep with a well-hung friend of ours on a regular basis. She likes to tell me how big he is and how she feels so incredible when he fucks her.
Early on in our relationship, it became clear that my size was a problem for her. We do love each other and sex - other than actual penetration with my cock - is great. But she needs more, and when I asked her about it, she was honest and told me that she was used to much larger and while she loved me, my cock wasn't big enough.
Frankly, the thought of her getting stretched out by his big cock turns me on no end.