@Siquall
Big disagree. If she took the time to think about how he's treated her all his life (as a close family friend), his current situation (needs to 'get married' but is clearly not ready for the actual thing), and the fact that he's dumb as a rock (which she is aware of), then she would probably come to the conclusion that he's not going to propose to her at that point.
The only thing the MC can be blamed on is being generally dense and dumb, but again, everyone around him is already aware of these traits. I haven't read the raws, but at this point in the story the FMC is definitely by far the most at fault. She verbally abuses him on a consistent basis for years, has sex with him once, and thinks he wants to marry her. Even if she didn't have the foresight to know (she should've if she's even an iota smarter than the MC), she still causes a scene and humiliates the MC for absolutely no reason. She's got the emotional maturity of a child.
I don't know why I wrote such a long comment for a Korean porn manhwa, but given that half of them are pretty much the same, this can more or less be applied elsewhere.