Determining whether something is sin is a lot like determining whether two chemicals mixed together are going to explode: it depends what you are mixing with what.
Some chemicals are prone to explode on their own, and some actions are bad news for whoever participates in them. Things like murder. That one’s pretty cut and dry.
But most chemicals only explode when mixed with specific other chemicals. In the same way, whether or not something is harmful differs from person to person. What is harmful for one person is just fine for another. What is fine in one situation is not fine in another.
For example, it was noble to lie to protect Jews from Nazis. But it is not noble to lie to protect myself from embarrassment.
Another example is porn. For some people, it adds to their intimacy with their partner and helps them explore their sexuality. For others, it fosters harmful fantasies and thoughts.
To make matters even more gray, there may be things that didn’t used to be okay for someone to engage in but they are now, and vice versa.
Actions and thoughts affect each person differently. That’s why there’s no point to judging others.