When people say "porn is necessary to prevent men from raping women in real life," they are implying that:
1. Women in porn act as a lightning rod for deviant sexuality; they take the hit so that women in the real world don't have to.
2. Masturbating to violence is an acceptable practice, as it helps a man refrain from raping women in reality.
3. Women in porn are not the women whom men rape in real life.
4. Pornography, as a phenomenon, is a tool for the mass management of male sexuality.
The most interesting part is the underlying assumption: that masturbation is a way to "process" rage and serves just as an "eco-friendly outlet" for the desire to rape someone.
There is no way that masturbating to pornography reinforces healthy sexuality or one's relationship with their own body. To me, 'healthy sexuality' is defined by both partners being recognized as subjects of action, rather than one (woman) being reduced to a mere object for the other to let off steam against.