Sin is anything that takes you away from God. Going away from God can most easily be defined as pulling away from love, justice, and compassion. This means that certain actions can be sinful at some times and not sinful at other times. For example, if you are having sex as a way to express trust, love and affection to another human being, it’s probably not a sin. If you are using sex to manipulate someone else’s feelings or to avoid your own problems, that might be sinful. The concept of intention as definition already exists in the morality of our legal code. If you purposely, willing end someone’s life it is murder. If you accidentally hit them with your car and they die, it is manslaughter. The first sin is hate, the second is negligence.
When discussing sin I tend to use words like “probably” and “might” because it is impossible for someone else to tell you if your action is sinful. They may give counsel, they may ask you to confront things you’ve been denying, but ultimately they can’t know your heart, your intentions, or your relationship with the Divine.