Leviticus 18 - Sexual Sin

There are many spiritual laws that are so great, that the breaking of them brings consequences that pour out into the physical world. Sexual Sins is one of them.

Fornication - Having sexual acts with someone else (either inside or outside of marriage).
Adultery - The breaking of a marriage covenant by having relationships outside of marriage.
Lasciviousness - Unbridaled lust; shamelessness; outrageousness.

I've had people tell me that they have never committed a sexual sin, because they have never been married, nor have they had physical relationships with those who were. They neglect the sin of Lasciviousness. God's Word says,

And he said, That which cometh out of the man, that defileth the man. For from within, out of the heart of men, proceed evil thoughts, adulteries, fornications, murders, Thefts, covetousness, wickedness, deceit, LASCIVIOUSNESS, an evil eye, blasphemy, pride, foolishness: All these evil things come from within, and defile the man. - Mark 7:20-23

And lest, when I come again, my God will humble me among you, and that I shall bewail many which have sinned already, and have not repented of the uncleanness and fornication and LASCIVIOUSNESS which they have committed - II Corinthians 12:21

Including Galatians 5:19; Ephesians 4:19; and I Peter 4:3

Matthew 5:27-28 takes it even a step further:
Ye have heard that it was said by them of old time, Thou shalt not commit adultery: But I say unto you, That whosoever looketh on a woman to lust after her hath committed adultery with her already in his heart.

We must continually keep our hearts pure before God, and always seek to live a holy life before God.