Porn tricks your brain to mix it with sex:
A growing body of scientific research involves the reward system of the brain. A reward system is a group of neural structures which help us learn from our environment by reinforcing behavioral responses to certain stimuli. These structures include circuits of neural pathways that developed to activate over the long course of evolution when we seek out things that benefit us.
When we encounter beneficial stimuli, our reward system gives us pleasurable feelings by releasing chemicals manufactured within the brain called neurotransmitters. Because we naturally seek more of these good sensations, we seek out more of the same stimuli that caused it in the first place; each additional time we encounter these stimuli, we release more neurotransmitters, get more pleasure, and reinforce our drive to yet again seek more of that stimuli.
Addiction causes a disorder of the reward system. Since the reward system is all about learning, researchers have called addiction a kind of "pathological learning." We learn to crave to a degree above what is healthy, to the point where we learn detrimental behaviors rather than ones beneficial for our survival.
Our brain is simple unable to distinguish from pornography and real sex. "Porn viewing, though not a substance, produces those same brain responses," says Dr. Mike Anderson, a sex and relationship expert. "If the viewing is accompanied by masturbation and orgasm — as it often is — the chemical and hormonal response is all the more complex and powerful, and the desire to repeat the behavior far stronger."
In part, the reward system includes our "upper brain" prefrontal cortex, which is responsible for making sophisticated decisions and distinctions. But the parts of the reward system involved in sensing pleasure and storing memory are in the "lower brain" limbic system.
Unfortunately, the lymbic system is not selective. It can't distinguish sexual intercourse from ejaculating to porn. To the lymbic system, if you have sensations of genital pleasure and climaxing to porn, it means that you are having sex and passing on your genes.