Parseltongue isn't any more unnatural than anything else. The unnatural thing is that Harry learnt it, he's not a born Parseltongue like the Lord Protector is.

Harry doesn't always get the words right. He's said that he has to concentrate hard because if you get it a little wrong the snake will stop listening to you.

He looked scared to me when he was doing it, like he was afraid he'd get the words wrong.