Strict answer: No, it is not a very major grave sin if there is truly no sexual desire, attachment, or arousal.

In Jain dharma, the main bondage comes from inner passions like lust, attachment, and intention, not merely from a physical act by itself. If you are only scratching for itching, cleaning, or relief, and there is no lustful thought, then it is not treated as a grave pāp.

But from a strict Jain discipline point of view:

• avoid unnecessary touching
• keep the mind pure, alert, and restrained
• if possible, manage it without indulgence or attachment

So the strict Jain answer is:

• With lust/desire: sinful and causes stronger bondage.
• Without lust/desire, only for relief: not a grave sin.

Still, it is best to remain careful and modest in conduct.