It's not really a crime... Dowan thought it was consensual and there wasn't a lot to make him think otherwise. there have been real life cases similar to this were none of the people involved could be charged with anything because of the fact that the person suffering from sexsomnia seems awake.
I agree. It would've been a crime if he didn't tell Chamin after realizing he has sexsomnia and continued having sex with him but that wasn't the case. He was just as devastated after all those times he thought Chamin was really having sex with him.
You said the "Dowan thought it was consensual" I also agree, but only on the first night. The other nights he knew chamin wasn't fully conscious and yet he continued. But whatever, there are more strange stories than that. ↁ_ↁ
Chamin doesn't know he's having sex with his roommate. I repeat, he's unconscious.
No matter which side I look at, I always see a crime.