Offering money to a girl to become a prostitute is not theft, and it's her choice if she agrees, but it's arguably immoral. What's her loss, something she would have given away for free anyway? Is it her innocence that's lost? It's debatable...

Sometimes matters of morality are complex and subtle, which is why a jury of 12 people is customary, and trials can take weeks.
Morality is a law, like the laws of physics, and we can't just make laws up.
Laws have to be discovered, they can't be invented.
Sometimes it takes a lot of work to uncover the real truth, both in science & law... It's worth doing though. :)