Its a concept, breaking a law that is supposed to have been created by a divine being.
To me the only sin you can commit is not being true to yourself. Screw religious laws, screw what the pope says, screw the church the mosque and whatever other religious building exists. You do what makes you feel good and if you need religion for that then good for you, but the second you let somebody else lead your life for you, you are committing a sin, not towards the church but towards god itself and towards you. Because you are ruining your life limiting your life, you are wasting the limited time you have trying to please something else. Please yourself, be happy live a life worth living.
Not doing that to me, is the only sin you can commit.