I know this is preaching to the choir, but here I go:
Some people wonder why that action or this thing is a sin, and here’s my understanding of why.
In everything theological, spiritual, biological, or whatever, in order to truly understand something, you gotta start with God and end with God, because He is the Beginning and the End of all good things. So to understand morality, we start with God.
God is Love and Truth. The Lord Himself tells us this in His Word. Because God is Love, that means He must oppose everything which is contrary to love; hence, lust, theft, envy, dishonor of father and mother are all offenses against God - and, speaking metaphorically, they wound God in the pupil of His eye. Because God is Truth, that means He must oppose everything which is contrary to truth; hence, adultery, idoltry, taking God’s Name in vain, homosexual acts are all offenses against God; and, again, speaking metaphorically, they wound God in the pupil of His eye.
A sin is an offense against God, so lust, theft, adultery, and idoltry are all sins.