We who have squandered the blessings we have are like Israel betraying God. This betrayal is deeply wicked. It is the sort of soul destroying wickedness that the prophet uses prostitution to establish the metaphor. It's not enough to betray a contract or covenant. It is like prostituting yourself.
The other message from this, is that some betrayals have consequences. Hosea doesn't end up living happily ever after with the prostitute. Her debasing herself meant that even after Hosea returned to her, they could not reestablish an untarnished relationship.