I have had the same view early on about masterbation... that it was OK as long as there were no lustful thoughts. To be completely honest... the fantasies have been very rare. I guess being a woman I don't struggle with that as much as men. In fact, my sister and I got on the subject and she brought up the verse in Matt about looking lustfully to which I replied if there are no lustful thoughts then it isn't a sin; she had no response. I don't recall having had this conversation with anyone else since, but I think God has been working this issue out in my heart. Here are some thoughts he has brought me through...
There is the verse about seed spilling onto idle ground... but then again, we have to work out the question of if this is relative or not. Meaning is this a general reference for all mankind or is this verse specifically speaking about men? Because if it specifically and literaly speaking of men... then that would imply that masterbation is sinful for a man, but not a woman. (I have totally lost my train of thought now cause I spent 15 minutes or so looking for that stinkin verse!!!)
But anyway, the bible does also say that a husband's body is not his own and a wife's body is not her own and and we are not to withhold it from eachother. I think that in order to remain pure for a future marriage it is important for me to think of my husband in an almost present tense sort of way in that I want to be the kind of godly wife that my (future) husband would desire to have. Along that line of thinking I try to consider how my choices now may effect my (future) marraige. For example, I have made some really sinful choices and have not remained celibate. I will have to deal with this sin from time to time for a very long time, if not the rest of my life on some levels. So if the choices we make today can effect our relationship with our future mate then it is important to consider what is best for that future relationship.
Now moving from the theological part of my point onto a biological one....
When I masterbate I am training my body to respond a certain way. to be more to the point... I have had this problem with masterbation for quite some time now, especially after my marriage was over. Since then I have had sex multiple times and I cannot recall once where I could reach an orgasm without supplementing my own stimulation. Now having come back to Christ and grown in spiritual maturity I realize it was a grave mistake to use the excuse that it isn't covered in the bible and I am not having lustful thoughts so it must not be sinful.
You are right, not everything is covered in the bible, but we have to look throughout the bible to see if we can interpret what would be said about a specific issue. I believe it is a sin to masturbate because we are taking away from our future marital sexual relationship that was meant as a means for worshiping God. Anytime we take anything away from worshiping God to its fullness is a sin.
I agree... if the only sin in the world was masterbation, we would have a much better world, but to say that belittles its magnitude. Sin is sin. Sin gets in the way of our relationship with God and should be dealt with not swept under the carpet because we think it is insignificant.
Does this mean I don't still struggle with this issue? absolutely not. Resolving something as sin in your mind doesn't make it any easier to deal with.