It's a lust issue, you have that picture in your head and are lusting after someone who isn't your husband, or worse still, you are fantasizing about and lusting after someone else's husband.
As it says in the Bible if you even look at a woman with lust you have already comitted adultery with her in your heart.

Yes, see, this is what I believe as well. It isn't the actual act, it is the thoughts that get you there.

If you could achieve orgasm without the dirty thoughts, then it wouldn't be a sin. I, however, am a pervert and cannot do it without truly sinful (sometimes even sick) thoughts.