Since Merlin wanted to be an English teacher he took classes for that while Arthur took business classes. One day Merlin went out for drinks with his classmates he sent out a text to Arthur telling him where he would be and what he would be doing. Arthur was furious, how dare he go out with other people without him. What If another person that wasn't him touched his merlin. No, he wouldn't allow it. 

When he was outside of the bar he heard a cry more specifically merlins cry. He went to the alleyway next to the bar and there he saw a crying drunk merlin with an unknown man. The man was touching merlin while merlin begged him not to. 

Arthur went towards the man and punched him until he passed out. He then took a crying Merlin in his hands and comforted him. He carried Merlin in bridal position and took him home while he cried. 

That night Arthur and Merlin slept in the same bed. Arthur holding Merlin closer than ever protecting him from the outside world. In the morning when they woke up they filled a lawsuit against the man for sexual harassment. Several months later they won the case and the man was sent to jail.