Is arousal and unintentional pleasure a sin? I was watching a film and became tempted. I was taken by surprise, was aroused and felt a sort of sexual pleasure, even though I turned away from any thoughts. I sat there helpless and had to experience this sort of stimulation and pleasure, even though I tried not to lust. Did I sin?
Gluttony is a sin. Yet, there are times when you smell something so particularly good that your mouth starts to water, even though you just recently ate and aren't the least bit hungry. The response of the body is automatic, basically out of your control; but what is in your control is your response. "Sure smells good, but I'm full. No thank you." No sin has occurred just because your body responded or that you were tempted to overeat.
In the same way, when your body identifies what it thinks is a sexual situation, it responds. You get aroused, your penis gets erect, and you start dripping pre-ejaculate fluid. All of these are automatic responses that feels good but are outside of your control. However, what you do is within your control. You can turn away from the source of arousal, you can walk out of the room, etc. Being aroused is not a sin, even when it feels good.
You didn't say what kind of film you were watching. I'm assuming it was not a pornographic video. Movies and other material that is designed to purposely arouse a guy is a sin to watch. "For this is the will of God, your sanctification; that is, that you abstain from sexual immorality; that each of you know how to possess his own vessel in sanctification and honor, not in lustful passion, like the Gentiles who do not know God" (I Thessalonians 4:3-5). "Lustful passion" refers to passionate things that are designed to arouse lust in the viewer -- i.e. pornography.