This topic has stirred up many more thoughts than I realized I had. The bottom line in every question of right or wrong is does the behavior express the love of God? Specifically, what is called in the Greek, agape love, which is a selfless love that cares more for the welfare of another than for oneself. Masturbation is selfish, not selfless.
I think just because the Greeks had a variety of names for love... eros for erotic love; phileo for friendship; etc ... there really is only one love, agape, which may be expressed through other forms. For example, just because Hebrews tells us that the marriage bed is holy and undefiled ( Heb. 13:4), that does not mean you can have any kind of sex with your wife. If she is reluctant and you force her using 1 Cor. 7 as a club, it is a violation of her will, thus remains rape. Likewise if you have intercourse without regard to her feelings, you are merely masturbating with her body. I have been married to my beloved for almost 25 years, and yet the Lord gave me a dream recently letting me know that my sexual behavior with her is selfish in 6 ways (I don't remember what they were, I only know I must be more considerate of her feelings in the future). I suspect that most wives tolerate their husbands fumbling with their bodies because they love them, but get little pleasure out of the duty.
There is a pastor in Denton, Texas who had gained renown for his expository study of the Love Song of Solomon. He acknowledges the metaphor of Christ's love for the church, but believes God really wanted to give direction on marital love. If you have a chance to listen to his tapes and study guide, it is an eye-opener on considerate love.
Sexuality is a gift with great pleasure, but I don't think many (myself included) have ever understood how to express agape love through erotic love. If one does not see the utter selfishness of masturbation, then I think those people are a long way from understanding what true godly love is.