It's not consensual. Enzo is holding Rian's friend hostage, so the only reason Rian goes along with it is because he has no real choice in that situation. In fact, he even tells Enzo to handcuff him because otherwise he might end up beating the sh*t out of him. That's a pretty clear indication that this isn't something he wants— it's something he's being forced to endure because Enzo has the upper hand.
So even tho Enzo ends up refusing to handcuff him because he wants to humiliate him and see him submitting "voluntarily," and Rian ends up playing along, it still isn't consensual in any meaningful sense of the word. Compliance under coercion is not consent.
Anyway, I agree with you, it's definitely not hot. There's nothing hot or sexy about watching someone suffer through it.
Also, Enzo either genuinely doesn't know how to have s*x (if force and self-gratification are all he's ever known), or he's simply too selfish, rotten, and consumed by his own desires to care whether his partner is actually enjoying it. Either way, it's pathetic.
Also, now that he told Rian he'll be using his ass from now on when having s*x, I hope Rian actually goes and finds someone else to use his ass with, just to show Enzo that not using his d*ck doesn't automatically mean he'll be having it with him !

No criticism to the style of sex, nor the author, nor the scene necessarily because clearly it's consensual.
But I'm still gonna clench my cheeks and look forward to some actual good bedding from them.
Enzo just seems like a bad lay as someone else said in the comments. Like damn at least make your partner feel good or smtg. I've read other "bdsm" BLs and GLs with much more cruelty and dubious concepts than this but at least both participants were enjoying the play. Here, it's literally just Enzo getting his rocks off and leaving my boy high and dry, with a quick ending to wrap things up and "assert dominance".
To each their own though, if you find it hot, then more power to you babes