Still I am wondering if masterbation is really a sin. My doubt stems from the following aspect:
(1) masterbation is a process quite primitive which we can reasonable believe is practiced by ancient people as long as the LORD creates them and makes such practice possible. I find no word mentioning ~{!0~}masterbation~{!1~} or similar words having the same meaning.
It is quite a contrast with a lot of words in the Moses~{!/~} five books when the LORD passes on people a lot of hygienic knowledge (not to quote those words for I am at office without a bible) in considerable details. Since masterbation is not only a matter of sexual stimulus, it is also a matter of hygience.
It occurs to me that the LORD is not any offended about this sort of behaviour; again it is a mystery why He did not mention it.
(2) ~{!0~}sin~{!1~}, as far as the biblical teaching is concerned, is ~{!0~}part of the LORD~{!/~}s creation~{!1~}. The meaning of ~{!0~}sin~{!1~} within the LORD~{!/~}s words is arguably not exhaustive.
If I agree with your argument about the ~{!0~}physical act of masterbation is a sin~{!1~}, LORD should have mentioned ~{!0~}masterbation is a sin~{!1~} quite clearly. As a substitute to this word, the words ~{!0~}promiscuous, lewded, filthy~{!-!1~} are mentioned from time to time, isn~{!/~}t it a bit peculiar the word ~{!0~}masterbation~{!1~} is the only word that is ~{!0~}exempted~{!1~} from mentioning, when it is so closely associated with the sin that the LORD mentioned so many times.
~{!0~}Sin~{!1~} can be a tool to the LORD. It is only a tool for the LORD to reveal himself. We can be reminded of the verses in Corinthian~{!/~}s or Ephesus~{!/~}s or somewhere (I can~{!/~}t remember), the created can~{!/~}t challenge the creator why he made the created this way or that way. (the parable of ~{!0~}clay pot~{!1~}). Another shining example is the happenings to Job when Satan bet with the LORD. I doubt this concept is popular throughout the whole bible.
There are a lot of behavior or thinking that the LORD find useless in terms of revealing himself; the Ten Commandments, however, are a lucid account of what ~{!0~}sins~{!1~} they are to reveal his glory. That~{!/~}s why it is reiterated from time to time, any time any where in the bible.
Yes, masterbation has nothing to do with such revelation. Surely, it is outside the scope of biblical teachings. It is done privately, it is not influencing anyone. To some extent, it helps subduing our burning passion that, on the contrary to what we~{!/~}ve been concluding, overcomes the temptation. Yes, what I said is a bit pragmatic, but the end results uphold what the LORD has commanded.
(3) ~{!0~}the physical act of masterbation~{!1~} is a natural use of our sex.
It is not a perverted use of our sex, not like those described as in people~{!/~}s act in Sodom and Gormorrah, not anything against the LORD~{!/~}s intention for the creation. The sin described in Roman~{!/~}s like people behaving against the natural use of their bodies - homosexuality and animal sex. Masterbation is beyond these sort of definition.
With all due respect, I still incline to think, if all sexual activities are subject to temptation and sin ( as I try to make good your definition of sexual fantasy being something mingled with temptation), we can~{!/~}t marry. We can~{!/~}t at all (as far as our scientific and biological knowledge assures us that marriage seemingly goes along with temptation before it can be a perfect marriage. {here I don~{!/~}t go too far to any divine implication of marriage I resort only to ethical teachings in the school and family planning organization}). After all, I am not saying ~{!0~}natural use of sex~{!1~} equals perfection. Just there is still un-resolvable conflict haunting our understanding between temptation, sin and creation.
Apparently there are far a lot of things that the LORD creates, having nothing to do with his revelation, but He allows them to go and exist. Some of these creations please the LORD because it reveals himself, some do not, (so we call them ~{!0~}sin~{!1~}), and, some do neither, which we can~{!/~}t find in Bible, but the LORD doesn~{!/~}t care, in terms of his relation with human beings.
(4) Irony of it all, your view of illicit things that tempt us lead us to do away from the LORD; it occurs to me, again, marriage should not go ahead; that~{!/~}s why Jesus~{!/~}s being in bachelor status and Mary giving birth to him in virginity put right with the perfection, if and only if, we keep virgin and do not marry (as Paul said and hoped, when he was speaking in the capacity of the LORD~{!/~}s servant, everyone ~{!.~}d better be single like him). Mankind should be confronted with the ~{!0~}end of the world~{!1~} if they strives to put themselves right with the LORD, because no marriage goes ahead (like Jesus), let alone descendants.
It is apparently very critical, for the Words to be fulfilled supposedly ~{!0~}to the end of the earth~{!1~}, it should be fulfilled almost exactly on the same day as the Judgement Day.