Because he's on a mission!! A honey trap. It's not like Rian is already in love with him.
I'm not sure Rian sees what happened to him as rape.
He frequently uses and views his body as a tool to get what he wants for his jobs.
It was shown previously that he would sleep with anyone so long as it helped his job or got him what he wanted.
At that moment in his mind, his job was to save his blonde coworker by useing his body to entice him to let him go.
It's also pretty clear to me that he's not afraid of Enzo at all, even after being forced like that. It also doesn't seem like he hates him for it either.
He might not have particularly liked it, but I don't think he hated it either. He seems ambivalent to it.
Also, on some level He wants to be closer to Enzo, I think that's why he let's him get away with doing whatever he wants to him.
Like the author suddenly wants us to forget the brutal rape one chapter back? Seriously? And why is Rian not escaping now he tried to do it before. Why is he helping Enzo, even after the horrible rape?