"Sin lies only in hurting others unnecessarily. All other sins are invented nonsense." -- Robert A. Heinlein
If you define sin as separation from the ways of God and His fellowship, I would disagree.
I do not have to "wrong" someone else directly to sin.
Here's an example.
If I view pornography, I am told that is wrong by the Church. Well, "who does that hurt", Mr. Heinlein might ask? It will hurt my wife. Think about it. If I have such a disorder, I begin to treat sex as a selfish outlet for only me. I don't care anymore about my wife and the intimate sharing that married love is supposed to be. It is no longer the total giving of myself to the other. In the long run, I am not fulfilling what God has planned for me, happiness here on earth and with Him in heaven.
Even solo sinning changes our attitudes towards others. It will eventually effect how we view our neighbor. It separates us from God's intention of our destiny and dignity.
Regards
If you do these things, you are hurting someone else unnecessarily. Heinlein's assertion holds true. He doesn't make any specific reference to whether something like viewing pornography is sinful or not - what he is saying is that anything can be sinful if it hurts others, nothing is if it doesn't hurt others (or yourself).
Granted, there can still be huge arguments about the definition of "hurting". ;)