Why even thinking of sinful acts is also sinful?

Because, thoughts will not definitely stop with that.

The thoughts will always be diverting you towards sinful acts.

Until you do them in action, the thoughts will always be driving you.

Even after doing in action, the thoughts will keep inducing you to do that in action again and again.

That is why, our scriptures beautifully say:

"Watch your thoughts, they become words.

Watch your words, they become actions.

Watch your actions, they become habits.

Watch your habits, they become your character.

Watch your character, it becomes your destiny."

How true it is!

So, if we like to avoid offenses, we should not even think of them.