Harry stared, looking back and forth between the two men, thrown completely off-kilter by their revelation. He had sworn never to tell a soul about his brief time with Regulus, who had only been helping Harry settle into his own body again.
Still, Sirius was gone and Harry wouldn't have wanted Regulus to be dishonest with Gaius if they had been sharing their pasts. Harry wasn't ashamed of sleeping with Regulus, but he did like to keep his private life private. He couldn't think of anyone he'd want to tell.
"Regulus saved me; he put me back in my body again, and I'll always be grateful," he said to Gaius with some gravity. In the next instant, though, he was meeting Regulus' eyes with a sly smile. "Even if I have lost my chance to return the favour."