Nah, she has a right to be upset.
They're both sex demons, so it's probably common for people of their species to go around hitting on each other (after all, they get pleasure and food at the same time). So she didn't do anything out of the ordinary when she hit on him.
Now, the problem is, Rei couldn't have sex with other demons the way she could. Whereas she was free, Rei had entered a contract with a witch. And sure, he wasn't aware to what consequences would follow if he cheated, but he knew that cheating on a witch would have SOME consequences.
So Rei was in the wrong for two reasons:
(a) He knowingly cheated.
(b) He didn't tell the other party that he had a contract with a witch, which could probably prove troublesome for the succubus as well.
Of course, if the story reveals that Rei made some passing remark like, "Oh btw I'm actually with a witch, hahaha," and then the succubus laughed it off, that would make this partially her fault as well, since that shows she has no qualms with sleeping with a taken man.
But as of now, there's nothing to indicate such, so I think it's safer to assume Rei didn't say anything, and the two just went straight to sex.
Why does that girl-guy demon keep sayin its his fault and wants revenge? Cuz she could have said NO Thanks instead of "your cute!,
I'll let you hit it "!!!!!!!!! Am I right? What a crazy bitch