To some of us certain activities are a sin, to others the exact same activities are not sinful. Each of us is different. Some Christians have no conscious when it comes to watching movies therefore they are not sinning by watching movies others however do. It is for you to decide how it makes you feel, if you have no conscious about watching movies then you are not committing a sin.
Even so viewing sin is mentally and spiritually damaging. The more someone watches things that they feel are ok the more likely they will go into more sinful things rather than less sinful things.
I do agree that with some activities and entertainment it is personal but other things whether they are a personal sin or not are still sinful and wicked and are best not to be done or observed. Be warned that trying to remove sin without the Holy Spirit's conviction of that sin will result in you doing things in your own strength and perhaps not paying attention to what God really wants to remove from you.
Ultimately scripture is our only source to judge the sinfulness of something. Here are some I've collected relating to this.
I will set no wicked thing before my eyes [Psalm 101:3]
Do you not know that friendship with the world is enmity with God? Therefore whoever wishes to be a friend of the world makes himself an enemy of God. [James 4:4]
Blessed is the man who does not walk in the counsel of the wicked or stand in the way of sinners or sit in the seat of mockers. [Psalm 1:1]
Do not conform any longer to the pattern of this world, but be transformed by the renewing of your mind. Then you will be able to test and approve what God's will ishis good, pleasing and perfect will. [Romans 12:2]
The lamp of the body is the eye. If therefore your eye is good, your whole body will be full of light. But if your eye is bad, your whole body will be full of darkness. If therefore the light that is in you is darkness, how great is that darkness! [Matthew 6:22-23]
Do not love the world or anything in the world. If anyone loves the world, the love of the Father is not in him. For everything in the worldthe cravings of sinful man, the lust of his eyes and the boasting of what he has and doescomes not from the Father but from the world.
[1 John 2:15-16]
Do you not know that your body is a temple of the Holy Spirit, who is in you, whom you have received from God? You are not your own; you were bought at a price. Therefore honor God with your body. (1 Corinthians 6:19-20)