When he finally admitted his feelings for her, she cried. He moved away, reluctant to cause her any further distress. She called out to him as he walked towards the door, her voice sounding odd in his ears. He'd never heard her cry.
"Where are you going?" she asked.
"Well... you obviously don't want me around –"
"Where did you get that stupid idea?"
"Maybe when you burst into tears the moment I admitted I loved you?" he asked incredulously. "Unless... Riza, do you–?!"
"Oh, Roy, you complete fool. Yes, I love you too. What took you so long?"
They'd agreed to keep it short. Neither one of them needed anything as complicated or as time-consuming as a relationship at that point in their lives. Certainly Ed wasn't ready for an emotional attachment.
So, nothing lasting. It's just... a release of tension, for the both of them. Ed is young, and often wound tight by the strenuous life he leads. Roy helps him to... unwind. And relax. And sleep without the nightmares.
They'd agreed to keep it short. But it didn't stop them from hoping the wait would be short, too, while Ed worked towards restoring his brother's body.