What a load of it...(not your fault, Skwim).Religious hard-liners extend their denunciation of any non-procreative sex to cover masturbation. Some of what I've quoted below from Bible.org is quite silly, but not unexpected.
Hmm, Seems to be unaware of mutual masturbation within marriage. Anyone surprised?"The Bible no where specifically forbids or denounces masturbation. It does, of course, denounce all forms of sexual impurity and fantasies that would involve adulterous relationships whether actual or mental.
It does warn against all forms of self-indulgence, adultery of the mind, and fornication. One of the problems involved is that though God created sex for enjoyment and pleasure, it is to be confined to the marriage relationship because it is also designed to express love (not just sexual love), unity, and total commitment. Masturbation is an act of self-gratification rather than a part of giving gratification and pleasure to one’s partner. Here are a few of the dangers of masturbation that some have listed:
1. Psychic effects. Masturbation has a tendency to isolate its captives psychologically and socially. In masturbation, the person is focused on self-alone even though he or she usually is fantasizing about someone else at the same time.
2. Emotional deprivation. It is impossible for the one who is practicing this habit to experience the full extent of sex emotions. Therefore, in short-circuiting the emotions one can easily be removed from the world of reality.
3. Damaged sensibility. The habit of masturbation has a tendency in numbing the mechanism of the sexual organs if practiced excessively. This lessens the sensibility and thus detracts from normal sexual relations of married life.
4. Self-gratification. The emotional background of self-gratification is not the least bit healthy and usually militates against the home, wife and family because it is focused only on self.
5. Control of the mind. Along with the act of masturbation comes the fantasy of the mind. When practiced often, a pattern or cycle seems to become established within the individual’s mind. Thus, perversion has a tendency to control the mind and this in turn initiates the act. The real danger lies in the guilt that increases as the individual dwells in this world of fantasy."
source
Think of this "full expression" and "full extent of sex emotions" in this way: sometimes, it's a beautiful thing to take your loved one to a full and wonderful dinner, with all the trimmings, and follow it up with the fullest expression of the sexual faculty possible."
And sometimes, you just want a snack.