You may have skipped too many steps here
Arya stopped laughing and said to Lith with a smile, "You've seen everything there is to see. Why would I be bothered if you saw it again once, twice or thrice? There's nothing left to hide anyway."
Immortals were crazy!
Lith had such a thought previously when he understood how boring life was for immortals and they sought out entertainment in every possible way. This time, however, he thought about it for a different reason.
Having a long life, there's a lot of experience accumulated and a person's attitude towards certain things also changes. If it were someone else in Arya's place right now, they would be embarrassed about it and would try to quickly hide their private regions after such a realisation.