In chapter 1, Tsubaki wanted Tadashi to ravish her and tried everything she could to make him do it. And she's never really stopped trying ever since. Her eternal, defining mistake is that she always thought if she could turn him on enough, he'd eventually lose control. That simply was never going to happen. The harder she tries to make it happen that way, the harder she's going to fail.
And that's actually a good thing. No, no, hear me out for just a moment. I'm not saying they shouldn't fuck. I actually tend to be entirely in favor of fictional characters doing the nasty in just about any constellation imaginable. It's just that Tsubaki can't be allowed to succeed in this specific way, given her backstory of experiencing sexual assault, and Tadashi being aware of it. Tadashi is fully committed to proving to her that men aren't animals -- that they're rational beings fully in control of their own actions, no matter how aroused, and therefore, men who commit sexual assault are, in fact, the scum of the Earth. And even if he's overthinking things, I can't say he doesn't have a point there.
Of course, now that Tsubaki seems to have finally accepted she's never going to get anywhere, it turns out her efforts will pay off in the end. She's gotten close enough to Tadashi that he can tell when she's putting on a brave face, and perhaps even connect the dots that if you reject your girlfriend's sexual advances once too often, she's going to lose heart just a little that she's ever going to get ravished.
