Yeah, you're probably right. Oh well, my poor attempt at humor caused more trouble than it was worth!
All of this is kind of off topic anyway. The point is whether or not masturbating is a sin, not if anything done in excess is a sin.
My personal opinion is this: There are lots of things that are not expressly forbidden in the Bible, however, the principles of the Bible are a good guide to follow in these so-called "grey areas." One principle that pops into my mind is "Abstain from all appearance of evil." My guess would be that the majority of masterbation is done with some kind of lust involved, most of the time for a complete stranger. The whole "I'm thinking about my spouse the whole time" argument just doesn't fly with me. If you're thinking about your spouse, why wouldn't you want to just actually have sex with them instead of just masterbating? They're not available? Have some self-control, then. That's one of the fruits of the Spirit. Even if you are just trying to think of your spouse the whole time, the devil is pretty tricky, and I bet he'll try to throw into your mind an image of someone who is not your spouse, at which point you're either going to keep going while you're thinking about that other person (sinful lust), or you're going to stop.... and how many times do you think anybody stops before they're done?
Getting back to the "abstain from all appearance of evil" idea...... Since the majority of masterbation is done while lusting after someone who is not your spouse, it's pretty likely that if someone sees someone else masterbating, they're going to think that they're doing it with someone who is not their spouse on their mind. Thus, it appears that they are committing adultery (according to Jesus - "if anyone thinks on a woman to lust after her, he has committed adultery"). And don't even think about saying "But nobody's ever seen me do it." Last time I checked, God was still omni-present.