Yeah, the story may seem a bit fast to some people. However, it's currently almost one month. People have fallen in love in that short amount of time. Men also fall in love faster and say I love you faster than women as well.
Their views toward one another changed throughout the story. As their views of each other added on, however, they just fell in love with each other more. (still doubting about the knowledge view...) Cauis was exhilarated that his magic has worked, his hard work into the summoning Kouichi, an extinct dwarf, who he considers at first glance beautiful. He was a bit too excited for several reasons, and like any seme in a manga/webtoon/etc., he almost had sex with him. He is normally kind, though, and apologized.
Kouichi, is similar to other people when they end up in an isekai world. Human beings are able to adapt pretty quickly. Compare his shocked scene to another one in an isekai manga. The only difference is that he's now a bride. He is more sensitive than others, and likes people who are kind. He tends to stay away from foreign things, and during that time the only one he actually has met and was introduced to is Caius. Cauis let's him roam with a bit of freedom, and their agreement is a bit understandable. After a few days, Kouichi had an oil massage. And he, like a teenager in a typical manga/webtoon/etc. was aroused. As a teenager, he's also curious about the world he's in and exploring it a bit. During this time he is forgetting about his life in Japan. I'd say he was affected by Cauis' kindness and was sad that he was lonely. The time that he says I love you was well timed by the author.
Three weeks in, they have mutual consent sex. After a ton of flirting by Cauis and the conflict that they went through, it is normal for the plot to be that way. There isn't a lot of story with Cauis flirting, and more sex bc of Kouichi's poisoning, but I like the sweeter parts of the story more. I'd say that the way they fell in love was a bit unusually fast, but it's normal in a yaoi. I'm not complaining.
I also want to hypothesize about the Karina fruit poisoning. I'm inclined towards the pregnant topic, but I'm still wondering how it's going to work out with the sex first. This is getting a bit long... Thanks for reading (>ω<)
I would like to say hi to your cat, hello! (*´∀`*)