People are both legitimately interested in and giggling about a recent health story regarding potential health benefits of masturbation. Studies indicate that prostate cancer risk may be reduced if one masturbates.
I need to address this question, for both the legitimately concerned and the gigglers, as part of proper spiritual formation, since Catholic teaching prohibits masturbation.
The Bible prohibits it in the form of forbidding of the spilling of seed. So the Bible is not commenting about sexual postures or actions, but it is the wasting of seed that is the issue, by engaging in a sex act that is closed to life. Remember that God is instructing people to not engage in sex acts that result in a closure to life, and are separate from the human component of a committed couple. This is why God prohibits the "spilling of seed," and not because wanking is a "bad" position or act per se. If so he would have also stated that women should not masturbate.
As I've repeatedly taught, you need to understand God's context. God's context is always biased in favor of life, and always prohibiting actions that will take the human family, both as a species and as individuals, to spiritual and literal dead ends.
When the Bible was written God was speaking to people who, like animals, had sex that was not separated from the procreation of children AND had society that enforced that anyone who wanted to be married, which was virtually everyone, they would be matched and married. (This is one reason there was polygamy, so that no woman would lack a husband and provider even if war or disease had depleted the male population). Therefore, there was no such thing as "pleasurable" or "recreational" sex. You did not have sex until you were married, and then you were encouraged to have a lot of sex and bear many children. This is the way that is most positive for life, and for the continuation of the human species as a whole, and also the family as the center of life and spirituality.
"Spilling of seed" would have been viewed as a man who shirked being married. It's not the masturbation per se that is the problem (like I said, God would have said so if it were, and made clear that it would apply to women too). Notice that he does not. God, by prohibiting male masturbation was prohibiting gratification by a man outside of his marital family context.
Nowadays we are in a society that has shattered families, and mocked those who are without families, and without love. Obviously the Catholic Church is correct to continue to uphold God's word in all matters. Likewise, though, one can have charity in understanding that we no longer live in the Biblical society where men and women were virtually guaranteed a marriage, where loneliness and lack of sex would not be an issue. So anyone who is prideful on the subject of masturbation nowadays is sneering up the wrong tree, and show their lack of understanding of what God taught, which was plain as dirt obvious to his chosen people. God never prohibited female masturbation, and he specifically addressed the issue of the spilling of seed to men and their duties. In fact, because human women do not have yearly or only a few times of years estrous cycles, like the animals, masturbation for women, and for men, would not be a problem if this is done in the actual act of married coupling in order to achieve the fullness of the act. In other words, the seed is not "spilled," but it is directed properly in the spousal relationship. Men are not to develop a lifestyle where that is considered the norm, either alone or outside of the marital relationship. God is micromanaging the "yes" or the "no" to the married open to life relationship, not micromanaging where the hands are placed and what they are doing, so long as it is heterosexual sex in a married couple open to life family plan.
So to wrap up, the Catholic Church is correct to continue to heed all of God's instructions, which are eternal and not "updated" as society mores change. But likewise the faithful must use some individual consciousness, charity, and common sense about what God intends in his direction. The Catholic Church is not mind control; it is quality control. It maintains the instructions of God, but also has a mechanism for achieving a fuller understanding. The Catholic Church keeps its priorities correct, which is the Ten Commandments and the Gospel of Jesus Christ. It does not want people to point and shout "wanker" to each other, when they don't even understand the pro-life and societal grounding of the culture of the Old Testament in the first place. I mean, use your brains please. God did not take Moses aside and say, "Oh, by the way, centuries from now humans will be so sinful that they will destroy the sanctity of life, the structure and heart of the family, and there will be lonely unmarried people, so write an appendix 'To be opened in the 1900's: it's OK to wank off if you are now trapped in a society where you cannot get a spouse.'" Sheesh.
I hope this helps.