Christians, is masturbation a sin?

I am married and when my wife is not around I tend to masturbate quite a bit though I still think about her when I masturbate even though I imagine her to look like a playboy playmate which she is clearly not. I am sorry but my wife is not the least bit attractive. I love her as a person though she is grossly overweight and turns me off sexually. I imagine her to look like Pamela Anderson with the same exact qualitys that my wife has. I even scream out her name when I climax. Is this wrong?