Grewd confirms the point exactly. You can't logically be a Christian, a true Christian, and practice fornication (i.e. including in thought and in heart if not in deed) at the same time and would have to choose one or the other. I'm beginning to come to terms that I can't have my cake and eat it to if I'm professing to be a Christian.
Now, for any Christian to practice this sin, they would have to believe either God doesn't care about that "pet sin", they will not really lose any rewards or they will not lose their salvation and go to hell, or are waiting for some magic moment when God delivers them out of the weakness and thus desire to commit that type of sin and pray this happens before they die. I don't find anywhere in the Bible where fornication is discussed "lightly", it's always met with "shall not inherit the kingdom of God". It did say to avoid fornication it's a good idea to marry, but, at a heart level, I have found that the aspect of fornication that is appealing is variety and one woman wouldn't cut it for that type of issue.
In my own experience I feel tormented in my mind everytime I masturbate. It's like something doesn't feel right about it and I have this pensive and sad sort of demeanor that comes on me afterwards as thought I did something wrong and am worried. This is not something I feel I can live with without taking "something" (i.e. like Tylenol) in order to manage some of these feelings. Now I recently masturbated just a few times. Twice to images of adult classifieds and once to just face pics (i.e. hijab women, fully covered no clothes). The face pics caused the biggest torment since it too perverse and I actually met a fully covered Muslim lady client just afterwards and felt awkward I did that. To me all that sounds a bit demonic and it's time to cut bait with this.