Read the beginning and the chapters. He saw her for the first time at that gathering where she talked back to her mom and that one lady. Then look at the beginning scene where the mc and ML are in the bedroom. Very likely he took interest because he probably is a masochist.
I thought the first chapter is where they are already married. I get that he got interested on her because she talked back to her mother but like marriage? Just because of that reason. There must be something else there.
The first chapter explained everything with images without spelling it out. He’s into being dominated in the bedroom. If you read and look at the reactions of his servants you see how they talk about if she’ll be able to handle him or his problem = his masochism. He witness her more dominant nature when talking back to her mother = his masochism felt that she would be perfect for him to let her dominate him.
And yes marriage, with stories like this- especially with smut, there is no normal logic. Many historical stories like this have marriages for simple reasons and they get married very quickly than modern stories. Even in real life people can get married after only a month of dating. Anyways the reason has been explained and it’s straight to the point, there’s nothing else suspicious because it’s clear this is not one of those plot heavy stories, it’s supposed to be a comedy and smut. This isn’t the first time a story has done this where one of the characters marries to satisfy their own pleasures.
How did she marry the crown prince? Like did he know her from the start? It's fishy really fishy!!!