There is nothing wrong if your employer fired you because of violations of the fraternization policy. Such policies are common in employment and are actually encouraged among employers. However, if you can prove that you were somehow fired because of the whistle-blowing itself, then you may have a claim for wrongful termination.
They say you get what you pay for, and this response is free, so take it for what it is worth. This is my opinion based on very limited information. My opinion should not be taken as legal advice. For true advice, we would require a confidential consultation where I would ask you questions and get your complete story. This is a public forum, so remember, nothing here is confidential. Nor am I your attorney. I do not know who you are and you have not hired me to provide any legal service. To do so would require us to meet and sign written retainer agreement. My responses are intended for general information only.
It depends on the nature of the policy.
A fraternization policy should prohibit romantic relationships between a manager and a reporting staff member. It should also define the behavior that is acceptable and what is not acceptable. The policy should also state clearly the consequences of breaking the policy. Idealy, the policy will provide courses of action that leave an employee with opportunities to understand and follow the policy.
This is my opinion and should not be construed as legal advise for your specific case as there are many more facts which you have not provided.