Porn, or porno (short of pornography) simply put is any type of media such as pictures, magazines, and video, which focuses on sex or sexual activity, often including nudity. There are varying degrees of how intense or graphic this can be, as well as many different genres or categories. For example, some porn videos try to be like a "real" movie with some sort of plot or storyline, and some are just one or more long glorified sex scenes.
You might be tempted to engage in viewing such media, typically for self arousal, however there are a number of things to consider before doing so which will hopefully convince you otherwise.
Unfortunately porn seems to have become more common or accepted in society. It is all too easy to search for or even accidentally come across such content on the Internet these days. As of March 2017, there are over 2,030,000,000 results when searching "porn" on Google. Imagine your underage child - all too tech savvy in this day and age - coming across such material. Or perhaps if you don't have kids imagine your parents or grandparents doing so. This thought probably would make you a bit uncomfortable at the very least.
Not to mention the morality aspect of watching porn. If doing something that is generally looked down upon, or where you feel has to be kept a secret (perhaps for fear of people judging you or considering you a pervert etc.), don't you think maybe it's likely not a very moral thing to be engaged in? When you do things that violate your morals you generally don't feel too good about yourself.
There is also the legality aspect where in some regions all forms of pornography are actually illegal, and underage porn is typically always illegal no matter the region. At times it may be hard to tell for sure if a "teen" is really 18 or 19, or if you're actually watching child pornography.
Also you may tend to get a portion of your mind stuck on what you watched even after some time has passed. When your attention is focused so, you may not really "be there", and this can negatively affect you at work or in your day-to-day life.
Watching such content also condones and endorses unethical behavior such as infidelity and promiscuity, both for the models/actors that take part in the pornography, and in the viewer's fantasies.
It can also lead to you becoming addicted, where you find yourself watching any chance you get, hiding it from people, and wasting time that you could otherwise be using productively.
It can also sort of put unreal or unnatural expectations towards your own sex life, such as not being "big" or "long" enough, or not lasting for hours, or having to perform in some bizzare fashion. You can also get to the point that it desensitizes you to real-life sex itself with your loved one, which is an important part of any sexual relationship. Perhaps you even can get so addicted that you feel you need to watch porn in order to get aroused at all.
There is also the theory that this habit can potentially have a similar effect of a drug addict where it is a gateway or stepping stone to further immoral and unethical actions, such as sexual abuse, promiscuity, rape, etc.
Websites containing pornographic content are notorious for infecting your computer/device with various computer viruses and other malware/spyware. You may end up in a situation where data on your computer is being stolen and/or deleted, your computer activity is being tracked, etc.
There are many free resources for viewing porn on the Internet, but many offer samples and lure you to pay for full content, either individual or subscription based. This can quickly lead to wasting money that could be well spent elsewhere, and supports this immoral and unethical industry.