Ok.. So if I harm no one through my sexual intercorse or if my girlfriend agrees on an open relationship than there is no sin, no? Or if I lie to protect someone? If I get drunk occasionally, smoke, do drugs?
If not, where is the problem, where is the problem with <<Sin>> .. Why does God warn to leave/punish us because of Sin?
The beginning of sin is sweet - the end is bitter. You probably haven't reached the end yet.
God doesn't punish anyone: you're merely reaping what you sow (like karma). If you use your girlfriend for (be it consensual) pleasure, a time will come when you will feel used and dumped too - so will she. Sooner or later, you become the slave of pleasure, hooked, unable to have a balanced, fruitful and accomplished existence.
If you lead a carefree life, you cast out the Holy Spirit and follow the flesh (your own ego, which separates you from God and other people). Your body instead of being a temple of the Spirit, becomes a den for robbers (evil spirits: lust, greed, pride, etc).
To know your own sin and sincerely repent for it is the most certain sign of the grace of the Spirit: "he that does truth comes to the light, that his deeds may be made manifest, that they are worked in God" (John 3:21).