The word "sin" originally meant "missing the mark" in archery or spear throwing. To sin was to hit the target, but miss the gold at the center.

If you miss the point, you sin. If you eat more than what the body can absorb effectively, it is gluttony and a sin. If you have sexual intercourse without accepting responsibility of raising the resulting children, it is lust and a sin. 

Do you get the point?