Sagawa must have deromanticised sex for herself due to her sleeping with boys who she didn't love and her thinking about how Chisaki is in relationships with people who aren't her when relieving herself. Her also using sexual contact to punish Ueno probably furthered the uglinesss of sex in her mind.
So when she finally got the chance to have a romantic and sexual experience with Chisaki, the person who she pined for throughout her middleschool to early university years, she didn't enjoy the deed. Her eyes being empty, like whenever she attacks Ueno, after the sexual encounter is proof of that.
She's lashing out at Ueno because he is the one person who she has power over and she probably hates herself for being like this. At the very least she hates that part of the reason why she can't connect to Chisaki is due to her own built up resentments.