No — not a very major grave sin, if there is truly no sexual desire and the act is only for relieving itching or hygiene.

In Jain dharma, bhav and intention are very important.

• Without lust/desire: it is not considered a grave sin.
• With lust, attachment, or enjoyment: it becomes sinful and harmful for the soul.

So the strictest Jain answer is:

• No, not a grave sin if done only for necessity.
• But it should still be done with restraint, cleanliness, and minimum touch.

If there is any possibility of lust or excitement, then it should be avoided.