Think about it like this, the guy you tried to kill almost got you killed and castrated and now you wake up to him having sex and enjoying it. Wouldn't it piss you off or disgust you?
But he's a selfish jerk and even a pervert... imagine trying to rape your cousin, lusting after them, that definitely spells pervert for me so instead of being pissed of or disgusted what if he's even enjoyin it?? Or what if he looked pissed off cuz he jealous someone else be doing uhm the mc?
The key point here is that Vasya almost castrated him and the cousin made his hate towards both of them obvious in the previous chapters, so if he still feels like lusting after Yuno then he is beyond helpless. Plus I'm pretty sure he won't live to tell the tale if he does get turned on since Vasya is also there. In anyway it's game over for him.
Omg... They seriously did it with the cousin just beside them!! Was that even a turn on?? That feels disgusting to me esp when that cousin tried to rape you????
Even if it wasnt a turn on and they just felt like so into it that they dont mind anything at all is just disturbing. Again, with how that f**ker beside them tried to rape him before and even tried to kill him!!
Oh well, each into his own really i guess...