Maybe I'm reading too much into things, but it seems like he's wanting Ripley to prioritize the wedding over the business because he's noticed how she's overworked herself. He clearly saw the bloodstain on her dress.
The "I set the Duchess's schedule" bit is toxic, though. Even if you can say he's using it as an excuse, I don't think it's right to do that. He can easily just say he knows she's pushing herself too hard, and is worried for her...
I like this manwha, or at least I want to, but there's certain aspects about this relationship that just seem bad. Between the aforementioned quote and forcing Ripley to stay inside... I'm not liking the Duke.
While you can also say that Ripley not taking the wedding seriously enough is bad, it does not justify this level of control he's trying to push on her. It's overbearing, and really gross.