It's fine to disagree. I happen to agree that something does not have to be explicitly identified as sin in Scripture for it to be sinful. I also happen to agree regarding chemicals and addiction. Dopamine and oxytocin are released with a wide variety of activities, any of which can become addictive, though very few are considered "sinful".
That said, I find that most people who hold a strong negative opinion on the subject
think that their position is clearly supported by Scripture, when in fact it is not. Challenge a person to find Scripture to support their opinion, and they either mention Onan (whose situation is vaguely relevant at best) or dodge and weave in discomfort.
Perhaps the better approach is to examine
why you think this particular activity is sinful, and whether your reasons for thinking such actually support your conclusion. It's one thing to consider the behaviour inappropriate or even to be personally disgusted by it, but such remain opinions, not objective facts. Our objective standard, the Bible, doesn't call it "sin" no matter how much we might think it does, or wish it did.
The solution is self-control, not condemnation. Trust the Holy Spirit to speak to others about their issues... and to you about yours.