It's not rape if he enjoys it.
That's not true is still rape, even if he likes it, even if he was aroused, he was not a willing participant even if it’s for the first second. (this is a work of fiction and we enjoy it as such, hey revenge of the nerds (a 1984 comedy) the leading couple end up together after he raped her, again a work of fiction that may or may not represent real events of any kind but nothing to live your life by).
In this case we can say many things but the phrase you used, it's a slippery slope, for bad behavior, and did they know he was going to enjoy it? Once he was naked strapped and then toyed, its rape.
So it's ok for me to enjoy it for what it is, sure! Should I bring morals to a piece of fiction I don’t think so. But you have to know the by today standards It’s rape.
Well that’s my take on it.