He doesn't like her, he's 'intrigued' at best and he's only marrying her because he has to marry a Langton.
Let's not start throwing rocks at MC just because we can see inside the ML's head.
If you already hate MC by chapter 12 then just drop this story, it's early enough that we haven't gotten into anything.
From what I understand the novel probably was written to make Adeline the 'perfect' match for ML. It was a romance novel after all and they always say that only the 'one' can be your true love.
So, even if he loves someone else according to the novel he'd fall in 'true' love with Adeline.
Hence why MC cannot trust ML or his feelings until Adeline comes into play and he actually rejects her.
This is just my theory though from the brief bits we've learned about the novel's destined love.