In his Confessions Book II, St. Augustine focusses on a number of adolescent sins. He obliquely suggests that he had sex out of marriage, but is more comfortable to confess stealing pears from a tree and then throwing those pears at some pigs. He wonders why do we sin? One problem was that his bodily urges produced such a cloud in his reasoning that he could not "distinquish the clear light of love from the murk of lust." Another, he says, is that he desired to imitate God's power, although his was a perverse imitation. He also cites to peer pressure, which in his case was also pear pressure, saying that if he were alone he never would have stolen the fruit. Indeed, he even recounts claiming to have done wicked things he did not do in order to avoid ridicule or to receive praise. See Book II. It is interesting how much his confession strikes me as the text of a "hip" teenage youth group lesson.
The idea of sin is problematic when it is used to declare that someone else deviate from your cultural normal is not only different from you, but in defiance of God. On the other hand, I think the notion of sin is very helpful for those of us who wish to live the best possible life. The idea that we periodically miss the mark makes us better.