Doesn’t I Corinthians 6:13 prove that masturbation is wrong?
How can you teach that masturbation is good for controlling the body when it clearly says in I Corinthians 6:13, "You say 'Food for the stomach and the stomach for food, and God will destroy them both.' The body, however, is not meant for sexual immorality but for the Lord, and the Lord for the body." This seems quite clear to me that the metaphor of eating when you are hungry is not directly analogous with sexual desire or masturbation. This has been one of your key arguments when dealing with this topic, but since Paul seems to directly contradict it, would you still recommend the practice?
The actual quote is: "Foods for the stomach and the stomach for foods, but God will destroy both it and them. Now the body is not for sexual immorality but for the Lord, and the Lord for the body" (I Corinthians 6:13). In context, Paul is addressing the topic of fornication. One argument made in favor of fornication is that God made people to be sexual, sex is a natural response to the desires that God gave us, so therefore all sex is allowable. Paul's response is that things of this life are temporary. You cannot use temporary things as the foundation of deciding eternal concepts of right and wrong. Yes, God made the body to eat and gave us hunger to remind us to eat. Does that mean I can eat whenever I want (which can become gluttony) or whatever I want (including stealing food from others)? By the same line of reasoning, God made people sexual and gave us a desire for sex, but God does not want His gift used to commit sin.
The difficulty you face with your argument is jumping to the assumption that masturbation is included in the Greek word porneia ("fornication", translated here as "sexual immorality"). See: Does "sexual immorality" (porneia) include masturbation? Paul's argument is that fornication cannot be justified just because God gave men sexual desires. Fornication is wrong because the Lord said we are not to commit fornication. Therefore, there is a requirement to prove that the act of masturbation (separate from pornography and lust) is condemned. As explained in Is masturbation unacceptable? the condemnation of the act is not present in the Bible, nor is it promoted. It is an act that is neither right nor wrong, but one that can be done in a sinful way, such as accompanied by pornography.
My argument regarding eating is solely about achieving self-control. People have an appetite whether they want it or not. You cannot control your appetite by not eating. Hunger will eventually drive you to eat and more likely than not you will overeat because you were not eating. For men, ejaculating is not an option. It will take place when the seminal vesicles get too full. Trying to control your temptation to look at pornography by not ejaculating will not work so long as the person ties the two in his mind. Understanding that pornography is not necessary to gain relief does give a person the opportunity to exercise self-control.