You're not wrong that there's nothing "objectively" wrong with it. Then again, killing might not be objectively wrong either. Aren't there circumstances under which killing a human being might not be considered objectively wrong? I'm sure we can think of some. And yet society generally considers homicide objectively wrong. Societies need to make decisions on what it will or won't tolerate. That can look very different depending on what society or culture you're talking about. Here in America, despite our basically over all low societal opinion of educators, we still don't generally want to see our teachers baring it all in front of a camera. And I hope that doesn't change. We expect our teachers to be positive influences on our children, and adult sex play is still considered verboten around children.