Yes, it's true you will not be physically hurting yourself or anybody else with cybersex or pornographic images. But there's more to relationships and happy families than just sex.
Virtual sex may have an inherent physical safety, but there are long term consequences for such instant gratifications.
And yes, there are some people who will obsess over things like sports so much that it destroys their relationships. But I hear about that much less often than about addictions ruining people's lives.
I'm sure you've heard many stories of alcohol and drug use ruining families or any sort of relationships. At first the substance abuse starts slow, and the user might think he has it under control. But these are highly addictive substances. There is very little chance that continued use will not lead to severe addiction and destroy any semblance of a good relationship and may also negatively impact several other areas of his life. Pornography (I include cybersex in that category) is also addictive. Just because it doesn't physically harm you immediately doesn't mean it doesn't harm you at all. The effect is largely emotional, mental, and spiritual, whatever you happen to believe about the spiritual aspect of life.
And as anybody should know who has taken a psychology class or just paid attention in life, your emotions can affect your physical health. Studies show that people who are happily married live on average 10 years longer than people who are single. People get sick when they are depressed. A great example would probably be the guy who wrote Peter Pan, James Barrie. When he was a young boy his older brother died and his mother was so distraught that she just stayed in her bed all day every day after that. When James would walk into the room, his mom would call him by his brother's name and when she found out it wasn't really him she would be very disappointed and cry. So he started wearing his brother's clothes and pretended he was his brother. Eventually James started getting too old and big to fit in the clothes and pull off being his brother, so he grew up and wrote about a land where children never aged. What does this have to do with physical health? Nothing, yet. He was a sickly person his whole life and when he died and was taken to the coroner to prepare for burial, they found out that his his body never fully matured. His growth was stunted; he was very short, only five feet tall. His reproductive organs were still like a child's, and his testicles hadn't even "dropped."
More info about him at Wikipedia
that has some hints on this.
Coming full circle, pornography is highly addictive and there is an extremely high chance that it will destroy a person emotionally. I don't know about anyone here, but I would much rather be physically harmed than emotionally or mentally. My body is not who I am; my thoughts and emotions are. If my thoughts and emotions get screwed up, what sort of a physical life would I be able to live? Surely you know (of) people who have severe emotional and/or mental problems and how they struggle to have an ordinary life.
The thing about addictions is that almost everyone thinks they have it under control until they try to quit or their life falls apart. Personally, I try stay away from addictive substances. I want to be in control of my life. I don't want anything controlling me. What I find strange is that in a time and society where the majority of the world seems to say do what you want because it's your life and you are free, I see a lot of people chained to their addictions. That's not freedom to me.