Yes, absolutely sinful. People, especially "church" people, don't like talking about this; but it's become an epidemic, and not just in the body of Christ but everywhere. There isn't a line, you should cut it out and ask the Holy Spirit to give you pure thoughts. This isn't a condemnation or criticism, just friendly advice from some who's been there.
Yes I agree
It is a sin..
You can not practice this act without evil thoughts.
Licentiousness is a sin.
Licentious = disregarding sexual restraints
Mar 7:22-23
22 Thefts, covetousness, wickedness, deceit, lasciviousness, an evil eye, blasphemy, pride, foolishness: 23 All these evil things come from within, and defile the man.
lasciviousness = licentiousness.
Rom 13:13 ISV Let’s behave decently, as people who live in the light of day. No wild parties, drunkenness, sexual immorality, promiscuity, quarreling, or jealousy!
Sexual immorality = unbridled lust, excess, licentiousness, lasciviousness, wantonness
Masterbation = Lust of the flesh
Rom 8:6-10
6 For to be carnally minded is death; but to be spiritually minded is life and peace. 7 Because the carnal mind is enmity against God: for it is not subject to the law of God, neither indeed can be. 8 So then they that are in the flesh cannot please God. 9 But ye are not in the flesh, but in the Spirit, if so be that the Spirit of God dwell in you. Now if any man have not the Spirit of Christ, he is none of his. 10 And if Christ be in you, the body is dead because of sin; but the Spirit is life because of righteousness.