Can you explain if it's unnatural for someone to be asexual?

It's fine: About 1% of adults of either gender whose bodies function normally simply feel that all this romantic stuff isn't for them. They need to be up-front and honest if they're seeking / pushed into relationships. Beyond this, somebody else's sexual orientation is their business, not yours. Most people no longer tolerate bashing gays as 'unnatural' and this should extend to the range of orientations.