Chrysoprasus is correct. Therein lies the dilemma, because I think you are both correct. However, I think DP is confusing what God views as marriage, and what we view as marriage; being marriage only AFTER a ceremony.
Hebraic culture wasn't like this, but then again, Pagan cultures held virginity in high estate as well. The greeks felt the virtues of virginity, when they clothed the vestals. Artemis is another example that comes readily to mind. Virginity has always been regarded as a high virtue, by virtually all cultures, at all times. It is only us "moderns" that have discovered a "sexual revolution." Well, of course you had the fertility cults of ancient time, now reviving as Wicca, but this sort of practice traces back to "demon copulation" in Babylon, as well as being "charmed" by priest, witch, sage, and magician. And this was always regarded as abominable, leading to such practices as child sacrifice, to placate the "gods" lust for virgins. Prostitution is regarded as the oldest profession in the world, because of these roots in magic and superstitions. I digress.
Back to the "sexual revolution."
Look at it's fruit all around us; in lives scarred and battered by sexual "addictions", abuses, divorces (with it's own tolls upon children and adults). Prostitution, slave trafficking (you think that sex is unrelated to this?) STD's, teen pregnancies, abortions, ad infinitum. Masterbation is sinful, because it is a cheapening of a sacred thing. Granted it is a rather "cheap" cheapening; I mean at least the worshippers of Molech were serious about sex's "power" of transcendence. Masterbation is only a cheapening of the soul, for want of recognizing anything transcending "animal nature" and " needs" in it. Sex is one of God's gifts to us that transcends ourselves and gives us a glimpse of something greater than ourselves, much like music, art, and poetry. It is "magic", but it is sacred because it is God's gift, not because it is "magic" in itself. This is why God considered the sex act a binding marriage vow. There is a mystery here, but it is clear enough to me, and all cultures, at all times in history, Pagan, Christian, or Atheistic, that sex, and virginity and marriage as it's 'consummation" (remember this word?) are sacred things. Masterbation is only a laughable imitation.
Frightening implications, don't you think?
Regards, Everett