What

They are in the same body, but they are not the same person

Think of it this way: while Ray is out, Sooha is unconscious as if he were asleep. Now imagine someone having their way with you while you're asleep. I think it's this kind of reasoning.

I think it goes more for the part of jealousy instead of someone “having their way” with them.
Sooha didn’t like the idea of Ray doing it with the seme first, hence he got jealous (at the very beginning). Now he would feel betrayed and cheated on if the seme did it with Ray, since Sooha and Ray have different consciousness and memories despite being inside of the same body.
So my point is, Sooha will actually feel hurt and cheated on instead of having the feeling that he got “harassed”