"But they replied, 'Should he have treated our sister like a prostitute?'"
Dinah was raped.
She is the only daughter of Jacob mentioned among his twelve sons, and she was raped by a man named Shechem, the local ruler. After raping her, Shechem "loved her" and "spoke tenderly" to her and desired to marry her. This isn't love. It's abuse.
Dinah's brothers burned with anger and were filled with grief over Dinah, their beloved sister. When Shechem and Hamor came to ask for Dinah's hand in marriage, the sons of Israel tricked them, agreeing only if every male in their household was circumcised. And while they were still recovering from the pain of circumcision, Simeon and Levi killed every last male in the city to avenge Dinah. They looted everything, women and children included. Jacob tells them they should not have done such a thing. Their response is in the verse above. The price of vengeance was generations of conflict between the Israelites and Canaanites and Perizzites.
This isn't a story on how to handle conflict or on forgiveness. They refused wealth, peace, and security for the sake of their sister's honor. We can argue about if what they did was right, but have you forgotten someone? This story is about Dinah. At the end of the day, there is a woman, bruised, broken, and traumatized from sexual abuse. Our sisters have been defiled. The question is, "What are we going to do about it?"