Wakana definitely treats her as if she was a plague, a larger than life contaminant. And I see the appeal of Sekai being this absolute evil, a true malevolent person that simply is (which definitely exists to some degree). However, there are two points to this.
At no point during her first events for this update Akira gave her the idea he wanted to have sex with her. In fact, he tries to leave multiple times. The whole conversation was about her forcing her points on him. When she offers sex, it's a direct response to believing that it's the only way to keep him with her. And that if he refuses, it means he's going to leave her. She's not
letting it happen, she's counting on it happening because in her mind it's the way things
have to go for him to remain with her. But that's
her idea, not Akira's.
Now, we're all adults, and from the reaction to the event, I think it's safe to say most of us understand that Io's out of her mind. The correct decision is not to engage in her damaged view of the situation, and to prove to her through actions that there is a third way. That way is not having sex with her and proceed to not abandoning her regardless. But we're not talking about how we view the situation, we're talking about how Io does.
I don't even see where we diverge so much in our take of the situation, because you basically explained exactly why she thinks this is a transaction but concluded by saying it's not a transaction. "I was already trash, and this person now knows the part of me I most despise, and didn't want him to find out. The only thing he's interested in is sex, that's the one thing I have left to give. Let's get it over with, this is the only path forward for him to stay with me, and for some reason, I can't bear not having him." That's her logic, it's her last resort transaction.