Consider these verses, DT:
1 Corinthians 6:18-2018 Flee fornication. Every sin that a man doeth is without the body; but he that committeth fornication sinneth against his own body. 19 What? know ye not that your body is the temple of the Holy Ghost which is in you, which ye have of God, and ye are not your own? 20 For ye are bought with a price: therefore glorify God in your body, and in your spirit, which are God's.
St. Paul says that the sexual sins are of a more serious nature.
Also, lust is involved which Our Lord clearly stated was sinful.
And think of how one's view of women would be changed by frequently indulging in this sin? They turn from sisters in Christ to mere sexual objects for mental fantasies.
Take those three together and it sounds pretty grave to me!