Actually, all physiological responses are due to "chemicals" like acetylcholine (a neurotransmitter) in the nervous system triggering receptors of the neurons to send a "signal" to the part of the body to make it work.

When your neural receptors are innundated with neurotransmitters, the neurons adapt by removing or altering the receptors. (this is how physical addiction works.. Nicotine receptors for instance tend to diminish when you smoke, causing you need to smoke more and more to get the same feeling or neural reaction.)

hence, because you were looking at porn excessively and causing those neurons to be bombarded with neurotransmitters, you probably don't have enough receptors left on your neural pathways to stimulate an action potential and signal the tissues of the penis to get erect. This does work in reverse, however. So relax and stop thinking about sex (easier said than done) for a while, and while they are starving for neurotransmitters your neurons will develop new receptors so that message can reach your penis.

That said, there is also some psychology that plays a big part in sexuality as well.

I'm not a medical doctor, but I am a graduate student in human physiology. This is all pretty much first year stuff... Go to wikipedia or just google and look up action potential and neurotransmitters for more info.