The dictionary defines sin as an immoral act considered to be a transgression against divine law I assume that divine law is also open to reason. The so called golden rule states (Matthew 7:12)"So in everything, do to others what you would have them do to you, for this sums up the Law and the Prophets."
Murder is a sin because you don't have right over another person's life in the same way as another person doesn't have right over your life. Stealing is sin because I don't have right over another person's property nor would I like anybody else to take what is mine, on the other hand smiling is not a sin because you don't encroach on another person's right when smiling and you may actually like being smiled upon.
Let me get to the point, in the old testament David(considered a prophet) did not sin when he engaged with concubines and slave girls who were not legally wives. If I define adultery as having sex with another man's wife, David has not broken the law. However today if two(unmarried) consenting adults have sex, that's adultery by biblical standards. From my understanding its encroaching on another person's right that seems to define sin.
Coming back to the question: Is sex between two consenting adults sin if so why. OR why would God mind when two people are having fun and not hurting anybody. Thanks.